Component for machine structural use and method for making the same

ABSTRACT

A component for machine structural use having improved fatigue strength. The component for machine structural use is produced by subjecting at least a part of a steel material to hardening, and the hardened structure is controlled such that the average grain diameter of former austenitic grains is 12 μm or less and the maximum grain diameter does not exceed four times the average grain diameter.

TECHNICAL FIELD

The present invention relates to components for machine structural use, components having hardened layers formed by high-frequency hardening in at least part of the components. Examples of the components for machine structural use described herein include drive shafts, input shafts, output shafts, crankshafts, inner and outer races of constant velocity joints, hubs, and gears for automotive vehicles.

BACKGROUND ART

Components for machine structural use, e.g., drive shafts and constant velocity joints for automotive vehicles, have been manufactured by the following method: A hot-rolled steel bar is processed into a predetermined shape by hot-forging, cutting, cold forging, or the like, followed by high-frequency hardening and tempering. In general, by this method, the fatigue strengths important for the components for machine structural use, e.g., torsional fatigue strength, bending fatigue strength, rolling fatigue strength, and sliding-rolling contact fatigue strength are attained.

Meanwhile, the demand for light-weight automobile components is on an increase in recent years because of the environmental concerns, and thus a further improvement on the fatigue strengths of automobile components is required.

There have been many proposed methods for improving the fatigue strength such as one described above.

For example, one possible approach for increasing the torsional fatigue strength is to increase the depth of hardening by the high-frequency hardening. Although the depth of hardening may be increased, the fatigue strength is saturated at a certain depth.

It is also effective to increase the grain boundary strength to increase the torsional fatigue strength. From this standpoint, a technology has been suggested which reduces the former austenitic grain diameter by dispersion of TiC. For example, the technology set forth in Patent Document 1 reduces the former austenitic grain diameter by dispersing a large amount of fine TiC during heating for high-frequency hardening. However, this technology requires solid solution treatment of TiC prior to the hardening and includes a hot-rolling step of heating the workpiece to a temperature of not less than 1,100° C. Thus, the heating temperature needs to be high during the hot rolling, and the production efficiency is disadvantageously low.

Moreover, the technology disclosed in Patent Document 1 above cannot sufficiently comply with the recent requirements on the fatigue strengths.

Patent Document 2 sets forth a shaft component for machine structural use, in which the torsional fatigue strength is increased by limiting the ratio (CD/R) of the depth CD of the hardened layer to the radius R of the shaft component subjected to the high-frequency hardening to 0.3 to 0.7 and then by adjusting the value A according to the amount of carbon, the value A being defined by the austenitic grain diameter γf within a depth of 1 mm from the surface after the high-frequency hardening, the average Vickers hardness Hf within (CD/R)=0.1 before the high-frequency hardening, and the average Vickers hardness Hc of the central portion of the shaft after the high-frequency hardening.

However, even when the CD/R is controlled, there is a limit to the improvement of the fatigue properties. This technology also fails to sufficiently comply with the recent requirement on the torsional fatigue strength.

Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-154819 (Claims)

Patent Document 2: Japanese Unexamined Patent Application Publication No. 8-53714 (Claims)

DISCLOSURE OF INVENTION

The present invention has been made based on the above-described situations and aims to provide a component for machine structural use with a higher fatigue strength and an advantageous method for manufacturing the component.

The inventors have vigorously pursued studies particularly on high-frequency hardened structures to effectively improve the above-described fatigue properties.

The inventors have focused attention on the grain diameter distribution of the former austenitic grains of a high-frequency hardened structure and found that the fatigue properties, such as torsional fatigue strength, bending fatigue strength, and rolling fatigue strength, can be improved by reducing the average grain diameter and the maximum grain diameter of the former austenitic grains.

The structure of the present invention can be summarized as follows:

1. A component for machine structural use, including a steel material at least part of which is subjected to hardening, wherein the hardened structure has an average diameter of former austenitic grains of 12 μm or less and a maximum grain diameter not exceeding four times the average grain diameter.

2. The component for machine structural use according to item 1, including a composition containing, in terms of percent by mass:

C: 0.3% to 1.5%

Si: 0.05% to 3.0%

Mn: 0.2% to 2.0%

and satisfying formula (1): C^(1/2)(1+0.7Si)(1+3Mn)>2.0   (1) wherein the balance is Fe and inevitable impurities.

3. The component for machine structural use according to item 2, the composition further containing, in terms of percent by mass:

Al: 0.25% or less.

4. The component for machine structural use according to item 2 or 3, the composition further containing, in terms of percent by mass, at least one selected from among:

Cr: 0.03% to 2.5%

Mo: 0.05% to 1.0%

Cu: 0.03% to 1.0%

Ni: 0.05% to 2.5%

Co: 0.01% to 1.0%

V: 0.01% to 0.5%

W: 0.005% to 1.0%

and satisfying formula (2): C^(1/2)(1+0.7Si)(1+3Mn)(1+2.1Cr)(1+3.0 Mo)(1+0.4 Cu)(1+0.3Ni)(1+5.0V)(1+0.5W)>2.0   (2) instead of formula (1).

5. The component for machine structural use according to item 2, 3, or 4, the composition containing, in terms of percent by mass, at least one selected from among:

Ti: 0.005% to 0.1%

Nb: 0.005% to 0.1%

Zr: 0.005% to 0.1%

B: 0.0003% to 0.01%

Ta: 0.02% to 0.5%

Hf: 0.02% to 0.5%

Sb: 0.005% to 0.015%

and satisfying formula (3): C^(1/2)(1+0.7Si)(1+3Mn)(1+2.1Cr)(1+3.0 Mo)(1+0.4 Cu)(1+0.3Ni)(1+5.0V)(1+1000B)(1+0.5W)>2.0   (3) instead of formula (1) or (2).

6. The component for machine structural use according to item 2, 3, 4, or 5, the composition further containing, in terms of percent by mass, at least one selected from among:

S: 0.1% or less

Pb: 0.01% to 0.1%

Bi: 0.01% to 0.1%

Se: 0.003% to 0.1%

Te: 0.003% to 0.1%

Ca: 0.0001% to 0.01%

Mg: 0.0001% to 0.01%

REM: 0.0001% to 0.1%.

7. A method for making a component for machine structural use, including subjecting at least part of a steel material to high-frequency heating at least once, the steel material containing one or both of a fine bainite structure and a fine martensite structure in a total of 10 percent by volume, wherein the high-frequency heating is conducted at a heating rate of at least 400° C./s and an ultimate temperature of 1,000° C. or less.

8. The method for making the component for machine structural use according to item 7, wherein the steel material is worked through: a hot-working step in which a total processing ratio at 800° C. to 1,000° C. is 80% or more; a cooling step subsequent to the hot-working step, in which the cooling rate is 0.2° C./s or more in the temperature range of 700° C. to 500° C.; and a secondary working step of either effecting 20% or more of working in a temperature range of 700° C. to less than 800° C. before the cooling step or effecting 20% or more of working in a temperature range of up to the A₁ transformation temperature subsequent to the cooling step.

9. The method for making the component for machine structural use according to item 7 or 8, wherein a retention time at 800° C. or more in the high-frequency heating is 5 seconds or less at a time.

10. The method for making the component for machine structural use according to any one of items 7 to 9, the steel material including, in terms of percent by mass,

C: 0.3% to 1.5%

Si: 0.05% to 3.0%

Mn: 0.2% to 2.0%

and satisfying formula (1): C^(1/2)(1+0.7Si)(1+3Mn)>2.0   (1) wherein the balance is Fe and inevitable impurities.

11. The method for making the component for machine structural use according to item 10, the steel material further including:

Al: 0.25% or less.

12. The method for making the component for machine structural use according to item 10 or 11, the steel material further including, in terms of percent by mass, at least one selected from among:

Cr: 0.03% to 2.5%

Mo: 0.05% to 1.0%

Cu: 0.03% to 1.0%

Ni: 0.05% to 2.5%

Co: 0.01% to 1.0%

V: 0.01% to 0.5%

W: 0.005% to 1.0%

and satisfying formula (2): C^(1/2)(1+0.7Si)(1+3Mn)(1+2.1Cr)(1+3.0 Mo)(1+0.4 Cu)(1+0.3Ni)(1+5.0V)(1+0.5W)>2.0   (2) instead of formula (1).

13. The method for making the component for machine structural use according to item 10, 11, or 12, the steel material further including, in terms of percent by mass, at least one selected from among:

Ti: 0.005% to 0.1%

Nb: 0.005% to 0.1%

Zr: 0.005% to 0.1%

B: 0.0003% to 0.01%

Ta: 0.02% to 0.5%

Hf: 0.02% to 0.5%

Sb: 0.005% to 0.015%

and satisfying formula (3): C^(1/2)(1+0.7Si)(1+3Mn)(1+2.1Cr)(1+3.0 Mo)(1+0.4 Cu)(1+0.3Ni)(1+5.0V)(1+1000B)(1+0.5W)>2.0   (3) instead of formula (1) or (2).

14. The method for making the component for machine structural use according to item 10, 11, 12, or 13, the steel material further including, in terms of percent by mass, at least one selected from among:

S: 0.1% or less

Pb: 0.01% to 0.1%

Bi: 0.01% to 0.1%

Se: 0.003% to 0.1%

Te: 0.003% to 0.1%

Ca: 0.0001% to 0.01%

Mg: 0.0001% to 0.01%

REM: 0.0001% to 0.1%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a representative example of a shaft.

FIG. 2 shows schemes of fatigue test and includes FIG. 2(a) showing a scheme of shaft torsional fatigue test and FIG. 2(b) showing a scheme of rolling fatigue test.

FIG. 3 is an optical microscope image of a hardened structure.

FIG. 4(a) and FIG. 4(b) are each a graph showing the relationship between the average former austenitic grain diameter and the torsional fatigue strength.

FIG. 5(a) and FIG. 5(b) are each a graph showing the effect of the former austenitic grain diameter and the maximum former austenitic grain diameter/average former austenitic grain diameter of the hardened layer on the torsional fatigue strength.

FIG. 6(a) and FIG. 6(b) are each a graph showing the effect of the processing ratio at temperatures lower than 800° C. and the conditions of the high-frequency hardening on the torsional fatigue strength.

FIG. 7(a) and FIG. 7(b) are each a graph showing the effect of the former austenitic grain diameter and the maximum former austenitic grain diameter/average former austenitic grain diameter of the hardened layer on the rolling fatigue strength.

FIG. 8 is a schematic diagram of a crankshaft.

FIG. 9 is a diagram showing the position of the crankshaft subjected to the high-frequency hardening.

FIG. 10 is a diagram showing the scheme of the endurance test.

FIG. 11 is a partial cross-sectional view of a constant velocity joint.

FIG. 12 is a cross-sectional view of a hardened structure in a constant velocity joint outer race.

FIG. 13 is another cross-sectional view of a hardened structure in a constant velocity joint outer race.

FIG. 14 is a partial cross-sectional view of a constant velocity joint.

FIG. 15 is a cross-sectional view of a hardened structure in a constant velocity joint inner race.

FIG. 16 is another cross-sectional view of a hardened structure in a constant velocity joint inner race.

FIG. 17 is a diagram showing a hub and a hub bearing unit.

FIG. 18 is another diagram showing a hub and a hub bearing unit.

FIG. 19 is a diagram showing the scheme of sliding rolling contact fatigue test.

FIG. 20 is a perspective view of a gear.

FIG. 21 is a cross-sectional view showing a hardened surface layer of tooth and bottoms of the gear.

Denotations of reference numerals in FIGS. 1 to 21 are as follows:

-   1: shaft -   2: spline -   3: chuck -   4: crankshaft -   5: journal -   6: crank pin -   7: crank web -   8: counter weight -   9: hardened structure layer -   10: drive shaft -   11: hub -   12: constant velocity joint -   13: outer race -   13 a: mouse -   13 b: stem -   14: inner race -   15: ball -   16: hardened structure layer -   17: hub -   18: shaft of hub -   19: spacer -   20: outer race of hub -   21: ball -   22: contact rolling surface -   23: fitting portion -   24: shaft of constant velocity joint -   25: gear -   26: teeth -   27: bottom -   28: hardened structure layer -   29: shaft hole

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described in specific details.

A component for machine structural use according to the present invention may take various shapes and structures, such as a drive shaft, an input shaft, an output shaft, a crankshaft, an inner or outer race of a constant velocity joint, a hub, and a gear for automobiles. In any case, it is essential that the component have a hardened layer over the entirety or part of the portion that particularly requires a fatigue strength, that the average grain diameter of the former austenitic grains be 12 μm or less, and that the maximum grain diameter does not exceed four times the average grain diameter.

The research results that led to this knowledge will now be explained.

A steel material (150 kg) having a composition indicated as steel a or steel b below was melted in a vacuum melting furnace- and hot-forged into a 150 mm square bar, from which dummy billets were made. Then, rolled steel bar materials were manufactured under various hot-working conditions.

[Steel a]: C: 0.48 mass %, Si: 0.55 mass %, Mn: 0.78 mass %, P: 0.011 mass %, S: 0.019 mass %, Al: 0.024 mass %, N: 0.0043 mass %, balance: Fe and inevitable impurities

[Steel b]: C: 0.48 mass %, Si: 0.51 mass %, Mn: 0.79 mass %, P: 0.011 mass %, S: 0.021 mass %, Al: 0.024 mass %, N: 0.0039 mass %, Mo: 0.45 mass %, Ti: 0.021 mass %, B: 0.0024 mass %, balance: Fe and inevitable impurities

Next, the steel bar was cut to a predetermined length and subjected to surface cutting and partly to cold-drawing to adjust the diameter. At the same time, the steel bar was rolled to form splines. A shaft 1 with splines 2 having a dimension and the shape shown in FIG. 1 was manufactured as a result.

The shaft was heated and hardened under various conditions in a high-frequency hardening device (frequency: 10 to 200 kHz), tempered in a heating furnace at 170° C. for 30 minutes, and then analyzed to determine the torsional fatigue strength.

The torsional fatigue strength was evaluated in terms of torque (N·m) when the number of cycles to fracture was 1×10⁵ in the shaft torsional fatigue test. In the torsional fatigue test, a hydraulic fatigue tester was used, and as shown in FIG. 2(a), splines 2 a and 2 b were respectively fit into disk-shaped chucks 3 a and 3 b and torsional torque was repeatedly applied between the chucks 3 a and 3 b at a frequency of 1 to 2 Hz.

The structure of the hardened layer of the same shaft was observed with an optical microscope to determine the average former austenitic grain diameter and the maximum former austenitic grain diameter.

The average former austenitic grain diameter was determined as follows. A specimen was observed with an optical microscope at three positions, namely, at ⅕, ½, and ⅘ of the thickness of the hardened layer from the surface, and from five field of views for each position under a magnification of ×400 (the area of one field of view: 0.25 mm×0.225 mm) to ×1,000 (the area of one field of view: 0.10 mm×0.09 mm). The average former austenitic grain diameter was measured at each position, and the highest average was defined as the average former austenitic grain diameter. The thickness of the hardened layer was assumed to be the depth of the region from the surface at which the area ratio of the martensitic structure decreased to 98%.

The maximum former austenitic grain diameter was determined as follows. A specimen was observed at the same three positions in the hardened layer thickness direction from five fields of view at each position under a magnification of ×400 (the area of one field of view: 0.25 mm×0.225 mm), so that the areas corresponding to the total of 15 fields of view can be observed. The grain diameter distributions in all the fields of view were applied to the equation below to determine the maximum grain diameter: Maximum grain diameter=average grain diameter+3σ (σ: standard deviation)

In observing the former austenitic grains, the cross-section taken in the thickness direction of the hardened layer was reacted with a corrosive liquid containing an aqueous picrinic acid solution (50 g of picrinic acid dissolved in 500 g of water), 11 g of sodium dodecylbenzenesulfonate, 1 g of ferrous chloride, and 1.5 g of oxalic acid so as to expose the former austenitic grain boundaries as shown in FIG. 3.

FIG. 4 shows the relationship between the average former austenitic grain diameter and the torsional fatigue strength. As shown in FIG. 4(a), the fatigue strength increased with the decrease in average grain diameter. However, at a former austenitic grain diameter of 12 μm or less, the fatigue strength differed although the grain diameter was substantially the same. It was found that the cause of this is the grain diameter distribution, in particular, the maximum grain diameter. Further investigations were pursued on this issue and it was found that at a maximum grain diameter of not exceeding four times the average grain diameter, the fatigue strength improves significantly by reducing the average grain diameter. Plotted points in FIG. 4(a) were re-plotted in FIG. 4(b) as open squares and rhombuses for a maximum grain diameter/average grain diameter of 4 or less and solid squares and rhombuses for a ratio grain diameter/average grain diameter exceeding 4.

The factors that cause the average grain diameter and the maximum grain diameter to affect the fatigue strength are presumably as follows.

The impurity elements that cause fatigue fracture tend to segregate at the former austenitic boundaries. Thus, as the former austenitic grains become smaller, the area in which segregation occurs increases. This decreases the impurity concentration in the individual segregation sites and thereby increases the fatigue strength. Furthermore, stress concentration to the former austenitic grain boundaries due to cutout or the like is dispersed when the grain diameter is small, thereby decreasing the stress applied onto the individual grain boundaries and increasing the fatigue strength. Such effects are affected not only by the average grain diameter but also by the maximum grain diameter. In other words, in the vicinity of a large grain, the area of the grain boundary is small and the impurity concentration easily increases. Moreover, dispersion of stresses does not readily occur.

It can be presumed that the presence of a large grain with a diameter exceeding four times the average grain diameter increases the possibility of decreasing the fatigue strength due to the above-described effects.

In particular, when the maximum grain diameter of the former austenitic grains is 20 μm or less, the fatigue strength is expected to improve significantly and stably for a wide range of component shape. More preferably, the average grain diameter is 5 μm or less. Most preferably the average grain diameter is 4 μm or less.

FIG. 5(a) and FIG. 5(b) are each a graph showing the effect of the average former austenitic grain diameter and the maximum former austenitic grain diameter/average former austenitic grain diameter in the hardened layer on the torsional fatigue strength. At an average former austenitic grain diameter of 12 μm or less, the fatigue strength can be notably increased by adjusting the maximum former austenitic grain diameter/average former austenitic grain diameter to 4 or less. Moreover, at an average austenitic grain diameter of 5 μm or less, particularly 3 μm or less, the fatigue strength can be further significantly increased by adjusting the maximum former austenitic grain diameter/average former austenitic grain diameter to 4 or less.

FIG. 6(a) and FIG. 6(b) show the effect of processing ratio at temperatures less than 800° C., the maximum ultimate temperature (heating temperature) during high-frequency heating, and the heating rate on the torsional fatigue strength. FIG. 6(a) and FIG. 6(b) show that excellent fatigue properties can be obtained at a processing ratio in the temperature zone less than 800° C. of 25% or more, a maximum ultimate temperature during the high-frequency hardening of 1,000° C. or less, and a heating rate of 400° C./s or more.

The effect of the average former austenitic grain diameter and the maximum former austenitic grain diameter/average former austenitic grain diameter on the rolling fatigue resistance was also studied. A steel material (150 kg) having a composition indicated as steel a or steel b above was melted in a vacuum melting furnace and hot-forged into a 150 mm square bar, from which dummy billets were manufactured. The billet was hot-worked and cold-drawn under various conditions and cut to make a steel bar 12 mm in diameter. The surface of the steel bar was subjected to high-frequency hardening under various conditions, and the resulting steel bar was cut to a predetermined length to make a specimen for rolling fatigue test. Radial-type rolling fatigue test shown in FIG. 2(b) was conducted using this specimen.

FIG. 7(a) and FIG. 7(b) show the results of the test. As with the torsional fatigue described above, the fatigue strength can be notably increased by adjusting the maximum former austenitic grain diameter/average former austenitic grain diameter to 4 or less at an average former austenitic grain diameter of 12 μm or less. Moreover, at an average austenitic grain diameter of 5 μm or less, particularly 3 μm or less, the fatigue strength can be further significantly increased by adjusting the maximum former austenitic grain diameter/average former austenitic grain diameter to 4 or less.

The experimental results used for obtaining FIG. 4(a), FIGS. 4(b) to FIG. 6(a), and FIG. 6(b) are set forth in Tables 1-1 and 1-2, and the results used for obtaining FIG. 7(a) and FIG. 7(b) are set forth in Tables 2-1 and 2-2. The rolling fatigue property is indicated as the ratio of the time to fracture to the time to fracture of Sample No. 1 in Table 2-1 which corresponds to an example of the existing related art. TABLE 1-1 Hot-working conditions Total Working HF hardening conditions processing Cooling (cold-working) Pre-hardening Retention ratio (%) at rate (° C./s) ratio (%) at structure Heating time (sec) at Steel 800° C. to after less than Proportion (vol. %) of Heating temperature 800° C. or No. type 1000° C. working 800° C. bainite structure rate(° C./s) (° C.) more  1 a 80 0.1 0 0 800 870 1  2 a 80 0.8 50 82 2000 870 1  3 a 80 0.8 40 82 800 870 1  4 a 80 0.8 25 82 2000 870 1  5 a 80 0.8 10 82 2000 870 1  6 a 80 0.8 0 82 2000 870 1  7 a 80 0.8 25 82 800 870 1  8 a 30 0.8 25 82 400 870 1  9 a 80 0.8 10 82 800 870 1 10 a 80 0.8 0 82 800 870 1 11 a 80 0.8 0 82 400 870 1 12 a 80 0.8 50 82 800 950 1 13 a 80 0.8 25 82 800 950 1 14 a 80 0.8 25 82 400 950 1 15 a 80 0.8 25 82 100 950 1 16 a 80 0.8 0 82 400 950 1 17 a 80 0.8 25 82 800 1050 1 18 a 80 0.8 10 82 800 1050 1 19 a 80 0.8 0 82 800 1050 1 20 a 80 0.8 0 82 100 1050 1 21 a 80 0.8 25 82 100 980 1 22 a 80 0.8 60 82 100 870 1 23 a 80 0.8 70 82 100 870 1 24 a 80 0.8 30 82 100 870 1 25 b 80 0.8 50 82 2000 870 1 26 b 80 0.8 40 82 800 870 1 27 b 80 0.8 25 82 2000 870 1 28 b 80 0.8 10 82 2000 870 1 29 b 80 0.8 0 82 2000 870 1 30 b 80 0.8 25 82 800 870 1 31 b 80 0.8 25 82 400 870 1 32 b 80 0.8 10 82 800 870 1 33 b 80 0.8 0 82 800 870 1 34 b 80 0.8 0 82 400 870 1 Hardened layer structure Torsional Average former Maximum former fatigue austenitic grain austenitic grain strength No. diameter (μm) diameter (μm) Max./Ave. (1 × 10⁵) (MPa) Reference  1 17 72 4.2 500 Rel. Ex.  2 4 8.7 2.2 844 Inv. Ex.  3 4.4 15 3.4 826 Inv. Ex.  4 4.8 18 3.8 808 Inv. Ex.  5 12.2 49 4.0 525 Comp. Ex.  6 13 53 4.1 520 Comp. Ex.  7 5.8 12 2.1 772 Inv. Ex.  8 6.2 18 2.9 754 Inv. Ex.  9 12.5 50 4.0 523 Comp. Ex. 10 13.9 60 4.3 512 Comp. Ex. 11 14 80 5.7 510 Comp. Ex. 12 8.5 16 1.9 727 Inv. Ex. 13 9.5 23 2.4 709 Inv. Ex. 14 9.6 35 3.6 673 Inv. Ex. 15 11 50 4.5 592 Comp. Ex. 16 13 61 4.7 500 Comp. Ex. 17 14 35 2.5 511 Comp. Ex. 18 15 60 4.0 502 Comp. Ex. 19 16 80 5.0 493 Comp. Ex. 20 15 120 8.0 484 Comp. Ex. 21 11.8 70 5.9 592 Comp. Ex. 22 4 18 4.5 620 Comp. Ex. 23 4.4 32 7.3 600 Comp. Ex. 24 8 40 5.0 600 Comp. Ex. 25 1.9 3.8 2.0 960 Inv. Ex. 26 2.4 8 3.3 940 Inv. Ex. 27 2.8 10.8 3.9 920 Inv. Ex. 28 2.6 14 5.4 750 Comp. Ex. 29 2.7 21 7.8 690 Comp. Ex. 30 3.9 7.9 2.0 880 Inv. Ex. 31 4.1 12 2.9 860 Inv. Ex. 32 4.2 18.5 4.4 730 Comp. Ex. 33 4.3 22 5.1 690 Comp. Ex. 34 4.4 26 5.9 680 Comp. Ex. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

TABLE 1-2 Hot-working conditions Total Working HF hardening conditions processing Cooling (cold-working) Pre-hardening Retention ratio (%) at rate (° C./s) ratio (%) at structure Heating time (sec) at Steel 800° C. to after less than Proportion (vol. %) of Heating temperature 800° C. or No. type 1000° C. working 800° C. bainite structure rate(° C./s) (° C.) more 35 b 80 0.8 50 82 800 1000 1 36 b 80 0.8 25 82 800 1020 1 37 b 80 0.8 25 82 400 1030 1 38 b 80 0.8 25 82 100 950 1 39 b 80 0.8 0 82 400 1030 1 40 b 80 0.8 25 82 800 1050 1 41 b 80 0.8 10 82 800 1050 1 42 b 80 0.8 0 82 800 1050 1 43 b 80 0.8 0 82 100 1050 1 44 b 80 0.8 50 82 800 950 1 45 b 80 0.8 25 82 800 950 1 46 b 80 0.8 25 82 400 950 1 Hardened layer structure Torsional Average former Maximum former fatigue austenitic grain austenitic grain strength(1 × 10⁵) No. diameter (μm) diameter (μm) Max./Ave. (MPa) Reference 35 6 11 1.8 800 Inv. Ex. 36 8.6 15 2.1 750 Inv. Ex. 37 10 30 3.5 700 Inv. Ex. 38 9 40 4.4 600 Comp. Ex. 39 9 48 5.3 595 Comp. Ex. 40 13 38 2.9 590 Comp. Ex. 41 14 56 4.0 580 Comp. Ex. 42 15 75 5.0 570 Comp. Ex. 43 14 110 7.9 560 Comp. Ex. 44 3 7.9 2.6 950 Inv. Ex. 45 5 11 2.2 810 Inv. Ex. 46 3.4 13 3.8 850 Inv. Ex. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

TABLE 2-1 Hot-working conditions Total Working HF hardening conditions processing Cooling (cold-working) Pre-hardening Retention ratio (%) at rate (° C./s) ratio (%) at structure Heating time (sec) at Steel 800° C. to after less than Proportion (vol. %) of Heating temperature 800° C. or No. type 1000° C. working 800° C. bainite structure rate(° C./s) (° C.) more  1 a 80 0.1 0 0 800 870 1  2 a 80 0.8 50 82 2000 870 1  3 a 80 0.8 40 82 800 870 1  4 a 80 0.8 25 82 2000 870 1  5 a 80 0.8 10 82 2000 870 1  6 a 80 0.8 0 82 2000 870 1  7 a 80 0.8 25 82 800 870 1  8 a 30 0.8 25 82 400 870 1  9 a 80 0.8 10 82 800 870 1 10 a 80 0.8 0 82 800 870 1 11 a 80 0.8 0 82 400 870 1 12 a 80 0.8 50 82 800 950 1 13 a 80 0.8 25 82 800 950 1 14 a 80 0.8 25 82 400 950 1 15 a 80 0.8 25 82 100 950 1 16 a 80 0.8 0 82 400 950 1 17 a 80 0.8 25 82 800 1050 1 18 a 80 0.8 10 82 800 1050 1 19 a 80 0.8 0 82 800 1050 1 20 a 80 0.8 0 82 100 1050 1 21 a 80 0.8 25 82 100 980 1 22 a 80 0.8 60 82 100 870 1 23 a 80 0.8 70 82 100 870 1 24 a 80 0.8 30 82 100 870 1 25 b 80 0.8 50 82 2000 870 1 26 b 80 0.8 40 82 800 870 1 27 b 80 0.8 25 82 2000 870 1 28 b 80 0.8 10 82 2000 870 1 29 b 80 0.8 0 82 2000 870 1 30 b 80 0.8 25 82 800 870 1 31 b 80 0.8 25 82 400 870 1 32 b 80 0.8 10 82 800 870 1 33 b 80 0.8 0 82 800 870 1 34 b 80 0.8 0 82 400 870 1 Hardened layer structure Average former Maximum former Rolling fatigue austenitic grain austenitic grain (ratio based on No. diameter (μm) diameter (μm) Max./Ave. related art) Reference  1 17 72 4.2 1.0 Rel. Ex.  2 4 8.7 2.2 10.0 Inv. Ex.  3 4.4 15 3.4 9.8 Inv. Ex.  4 4.8 18 3.8 9.5 Inv. Ex.  5 12.2 49 4.0 1.5 Comp. Ex.  6 13 53 4.1 1.4 Comp. Ex.  7 5.8 12 2.1 6.4 Inv. Ex.  8 6.2 18 2.9 6.3 Inv. Ex.  9 12.5 50 4.0 1.5 Comp. Ex. 10 13.9 60 4.3 1.4 Comp. Ex. 11 14 80 5.7 1.3 Comp. Ex. 12 8.5 16 1.9 4.4 Inv. Ex. 13 9.5 23 2.4 4.2 Inv. Ex. 14 9.6 35 3.6 3.4 Inv. Ex. 15 11 50 4.5 2.2 Comp. Ex. 16 13 61 4.7 1.0 Comp. Ex. 17 14 35 2.5 1.4 Comp. Ex. 18 15 60 4.0 1.1 Comp. Ex. 19 16 80 5.0 0.9 Comp. Ex. 20 15 120 8.0 0.9 Comp. Ex. 21 11.8 70 5.9 2.2 Comp. Ex. 22 4 18 4.5 3.0 Comp. Ex. 23 4.4 32 7.3 2.9 Comp. Ex. 24 8 40 5.0 2.1 Comp. Ex. 25 1.9 3.8 2.0 12.1 Inv. Ex. 26 2.4 8 3.3 12.0 Inv. Ex. 27 2.8 10.8 3.9 11.0 Inv. Ex. 28 2.6 14 5.4 6.5 Comp. Ex. 29 2.7 21 7.8 5.5 Comp. Ex. 30 3.9 7.9 2.0 10.7 Inv. Ex. 31 4.1 12 2.9 10.8 Inv. Ex. 32 4.2 18.5 4.4 4.5 Comp. Ex. 33 4.3 22 5.1 4.3 Comp. Ex. 34 4.4 26 5.9 4.3 Comp. Ex. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

TABLE 2-2 Hot-working conditions HF hardening conditions Total Working Retention processing (cold- Pre-hardening time ratio (%) at Cooling rate working) ratio structure (sec) at Steel 800° C. to (° C./s) after (%) at less Proportion (vol. %) of Heating rate Heating 800° C. or No. type 1000° C. working than 800° C. bainite structure (° C./s) temperature(° C.) more 35 b 80 0.8 50 82 800 1000 1 36 b 80 0.8 25 82 800 1020 1 37 b 80 0.8 25 82 400 1030 1 38 b 80 0.8 25 82 100 950 1 39 b 80 0.8 0 82 400 1030 1 40 b 80 0.8 25 82 800 1050 1 41 b 80 0.8 10 82 800 1050 1 42 b 80 0.8 0 82 800 1050 1 43 b 80 0.8 0 82 100 1050 1 44 b 80 0.8 50 82 800 950 1 45 b 80 0.8 25 82 800 950 1 46 b 80 0.8 25 82 400 950 1 Hardened layer structure Rolling Average former Maximum former fatigue (ratio austenitic grain austenitic grain based on No. diameter (μm) diameter (μm) Max./Ave. related art) Reference 35 6 11 1.8 10.4 Inv. Ex. 36 8.6 15 2.1 10.2 Inv. Ex. 37 10 30 3.5 9.9 Inv. Ex. 38 9 40 4.4 3.5 Comp. Ex. 39 9 48 5.3 3.5 Comp. Ex. 40 13 38 2.9 2.3 Comp. Ex. 41 14 56 4.0 2.6 Comp. Ex. 42 15 75 5.0 2.3 Comp. Ex. 43 14 110 7.9 2.4 Comp. Ex. 44 3 7.9 2.6 10.6 Inv. Ex. 45 5 11 2.2 10.3 Inv. Ex. 46 3.4 13 3.8 10.8 Inv. Ex. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

In order for the average grain diameter of former austenitic grains to be 12 μm or less and for the maximum grain diameter to be not more than four times the average grain diameter, it is advantageously suitable to use an approach of allowing the structure before high-frequency hardening to contain homogeneous, fine bainite structure and/or martensite structure. This approach is described below.

That is, the structure before the high-frequency hardening preferably contains 10 vol % or more and more preferably 25 vol % or more of a bainite structure and/or a martensite structure. Since the bainite structure and the martensite structure are each a structure containing finely dispersed carbides, inclusion of large amounts of bainite structure or martensite structure in the pre-hardening structure will increase the area of the ferrite/carbide boundaries, which are nucleation sites for austenite, during hardening by heating and finer austenite is generated as a result. Thus, this approach effectively contributes to reducing the former austenitic grain diameter of the hardened layer. Since the austenitic grain diameter decreases during the hardening by heating, the grain boundary strength increases and the fatigue strength improves.

In order to adjust the proportion of the homogeneous, fine bainite structure and/or martensite structure to 10 vol % or more, a steel having the composition described below is preferably hot-worked such that the total processing ratio at temperatures of 800° C. to 1,000° C. is 80% or more, followed by cooling at a cooling rate of 0.2° C./s or more for the temperature range of 700° C. to 500° C. This is because, at a total processing ratio at 800° C. to 1,000° C. of less than 80%, a sufficiently homogenous, fine bainite or martensite structure cannot be obtained. The total proportion of the bainite structure and/or the martensite structure cannot be adjusted to 10 vol % or more unless the cooling rate in the temperature range of 700° C. to 500° C. is 0.2° C./s or more after the hot-working.

Moreover, in order to reduce the average grain diameter and the maximum grain diameter of former austenite of the hardened layer after the high-frequency hardening, it is necessary to effect 20% or more of working (second working step) in the temperature range less than 800° C. before the high-frequency hardening. The working in the temperature range less than 800° C. may be effected in the hot-working step before the cooling at the above-described cooling rate, i.e., may be effected in the temperature range of 700° C. to less than 800° C. Alternatively, a separate cold-working may be effected after the cooling or a hot-working may be effected by heating the workpiece at the A₁ transformation temperature or less. The processing ratio at temperatures lower than 800° C. is more preferably 30% or more.

Examples of the working method include cold forging, cold ironing, forming rolling, and shotblasting.

A preferable steel composition for obtaining such a pre-structure will now be described.

-   C: 0.3 to 1.5 mass %

Carbon is the element having the greatest influence on the hardenability. Carbon effectively contributes to increasing the hardness and the depth of the hardened layer and to improving the fatigue strength. However, at a C content less than 0.3 mass %, the depth of the hardened layer must be significantly increased to securely achieve the required fatigue strength, and this leads to occurrence of significant quenching cracks and difficulties in generating the bainite structure. Thus, 0.3 mass % or more of carbon is contained. At a C content exceeding 1.5 mass %, the grain boundary strength decreases, the fatigue strength decreases, and the machinability, cold-forgeability, and resistance to quenching crack decrease. Therefore, the C content is limited to the range of 0.3 to 1.5 mass %, preferably 0.4 to 0.6 mass %.

-   Si: 3.0 mass % or less

Silicon not only serves as a deoxidizer but also contributes to effectively improving the strength. At a Si content exceeding 3.0 mass %, the machinability and the forgeability decrease. Thus, the Si content is preferably 3.0 mass % or less.

In order to increase the strength, Si content is preferably 0.05 mass % or more.

-   Mn: 2.0 mass % or less

Manganese is added for its effects of improving the hardenability and securing the depth of the hardened layer during the hardening. At a Mn content less than 0.2 mass %, the effect of addition is not sufficient. The Mn content is preferably 0.2 mass % or more, and more preferably 0.3 mass % or more. On the other hand, at a Mn content exceeding 2.0 mass %, the residual austenite after hardening increases, thereby decreasing the surface hardness and the fatigue strength. Thus, the Mn content is preferably 2.0 mass % or less. At an excessively large Mn content, the base material hardens and the machinability may be adversely affected. Thus, the Mn content is preferably 1.2 mass % or less and more preferably 1.0 mass % or less.

Al: 0.25 mass % or less

Aluminum is effective for deoxidization. It is also effective for reducing the grain diameter of the hardened layer since it restrains the growth of the austenitic grains during the heating for hardening. At an Al content exceeding 0.25 mass %, however, the effect is saturated and the cost for the composition is disadvantageously increased. Thus, the Al content is preferably 0.25 mass % or less, and more preferably 0.001 to 0.10 mass %.

In the present invention, the above-described four components are the main components. The main components must satisfy formula (1) below: C^(1/2)(1+0.7Si)(1+3Mn)>2.0   (1)

By adjusting the C, Si, and Mn contents to satisfy formula (1), it becomes possible to bring the total proportion of the bainite and martensite in the pre-high-frequency hardening structure to 10 vol % or more and to produce a hardened layer having the structure of the present invention by the high-frequency hardening. Moreover, when the value of the left side of formula (1) is 2.0 or less, the hardness of the hardened layer after the high-frequency hardening is low, and it becomes difficult to sufficiently maintain the depth of the hardened layer.

Although the main components are described above, at least one of the following six components may also be incorporated according to the present invention.

-   Cr: 2.5 mass % or more

Chromium is effective for improving the hardenability and for ensuring the hardening depth. An excessive amount of Cr stabilizes the carbides and promotes generation of the residual carbides, thereby decreasing the grain boundary strength and the fatigue strength. Accordingly, the Cr content is preferably as low as possible, but can be up to 2.5 mass %. Preferably, the Cr content is 1.5 mass % or less. In order to improve the hardenability, the Cr content is preferably 0.03 mass % or more.

-   Mo: 1.0 mass % or less

Molybdenum is effective for inhibiting the growth of the austenitic grains. For this effect, the Mo content is preferably 0.05 mass % or more. At a Mo content exceeding 1.0 mass %, the machinability is degraded. The Mo content is preferably 1.0 mass % or less.

-   Cu: 1.0 mass % or less

Copper is effective for improving the hardenability. Copper forms solid solution in the ferrite, and this solid-solution hardening increases the fatigue strength. Copper also inhibits generation carbides, prevents a decrease in grain boundary strength caused by the carbide, and improves the fatigue strength. However, at a Cu content exceeding 1.0 mass %, cracks occur during hot-working. Thus, the Cu content is preferably 1.0 mass % or less, and more preferably 0.5 mass % or less. At a Cu content less than 0.03 mass %, the effect of improving the hardenability and the effect of preventing the decrease in grain boundary strength are small. Thus, the Cu content is preferably 0.03 mass % or more.

-   Ni: 2.5 mass % or less

Nickel improves the hardenability and is used to adjust the hardenability. It also inhibits generation of carbides, prevents a decrease in grain boundary strength caused by carbides, and increases the fatigue strength. However, Ni is very expensive and increases the cost of steel material if added in an amount exceeding 2.5 mass %. Thus, the Ni content is preferably 2.5 mass % or less. At an Ni content less than 0.05 mass %, the effect of improving the hardenability and inhibiting the decrease in brain boundary strength are small. Thus, the Ni content is preferably 0.05 mass % or more, and more preferably 0.1 to 1.0 mass %.

-   Co: 1.0 mass % or less

Cobalt inhibits generation of carbides, inhibits a decrease in grain boundary strength caused by carbides, and increases the fatigue strength. However, Co is a very expensive element, and incorporation of 1.0 mass % or more of Co will increase the cost of the steel material. Thus, the Co content is 1.0 mass % or less. At a Co content less than 0.01 mass %, the effect of inhibiting the decrease in grain boundary strength is small. Thus, the Co content is preferably 0.01 mass % or more and more preferably 0.02 to 0.5 mass %.

-   V: 0.5 mass % or less

Vanadium bonds with C and N in the steel to function as a deposition strengthening element. It also increases the resistance to temper softening. These effects increase the fatigue strength. At a V content exceeding 0.5 mass %, these effects are saturated. Thus, the V content is preferably 0.5 mass % or less. At a V content of less than 0.01 mass %, the effect of improving the fatigue strength is small; thus, the V content is preferably 0.01 mass % or more and more preferably 0.03 to. 0.3 mass %.

-   W: 1.0 mass % or less

Tungsten is useful for inhibiting the growth of austenitic grains. For this effect, the W content is preferably 0.005 mass % or more. At a W content exceeding 1.0 mass %, the machinability will be degraded. Thus, the W content is preferably 1.0 mass % or less.

In adding one or more of the six components to the main components, formula (2) below must be satisfied for the same reason as for formula (1): C^(1/2)(1+0.7Si)(1+3Mn)(1+2.1Cr)(1+3.0 Mo)(1+0.4 Cu)(1+0.3Ni)(1+5.0V)(1+0.5W)>2.0   (2)

In the present invention, at least one selected from Ti: 0.1 mass % or less, Nb: 0.1 mass % or less, Zr: 0.1 mass % or less, B: 0.01 mass % or less, Ta: 0.5 mass % or less, Hf: 0.5 mass % or less, and Sb: 0.015 mass % or less may be further contained.

-   Ti: 0.1 mass % or less

Titanium bonds with N, which is an inevitable impurity, and prevents B from forming BN and from losing the effect of increasing the hardenability, thereby fully enhancing the hardenability increasing effect of B. To achieve this effect, the Ti content is preferably 0.005 mass % or more. At a Ti content exceeding 0.1 mass %, large amounts of TiN are generated, and this starts fatigue fracture and thus causes a significant decrease in fatigue strength. Thus, the Ti content is preferably 0.1 mass % or less and more preferably in the range of 0.01 to 0.07 mass %.

-   Nb: 0.1 mass % or less

Niobium not only has hardenability enhancing effect but also serves as a deposition strengthening element by bonding with C and N in the steel. It is also an element that increases the temper softening resistance. These effects improve the fatigue strength. However, at a Nb content exceeding 0.1 mass %, these effects are saturated. Thus, the Nb content is preferably 0.1 mass % or less. At a Nb content less than 0.005 mass %, the effect of deposition strengthening and the effect of enhancing the temper softening resistance are small; thus, the Nb content is preferably 0.005 mass % or more, and more preferably 0.01 to 0.05 mass %.

-   Zr: 0.1 mass % or less

Zirconium not only has a hardenability improvement effect but also serves as a deposition strengthening element by bonding with C and N in the steel. It is also an element that increases the temper softening resistance. These effects improve the fatigue strength. However, at a Zr content exceeding 0.1 mass %, these effects are saturated. Thus, the Zr content is preferably 0.1 mass % or less. At a Zr content less than 0.005 mass %, the effect of deposition strengthening and the effect of enhancing the temper softening resistance are small; thus, the Zr content is preferably 0.005 mass % or more, and more preferably 0.01 to 0.05 mass %.

-   B: 0.01 mass % or less

Boron not only improves the fatigue properties by strengthening the grain boundaries but also improves the strength. The B content is preferably 0.0003 mass % or more. Since the effect is saturated at a content exceeding 0.01 mass %, the B content is limited to 0.01 mass % or less.

-   Ta: 0.5 mass % or less

Tantalum may be contained since it is effective against delay of microstructural changes and prevents degradation of fatigue strength, in particular, rolling fatigue strength. However, Ta does not contribute to further strength improvements even when the Ta content is increased over 0.5 mass %. Thus, the Ta content is set to 0.5 mass % or less. In order to achieve the effect of improving the fatigue strength, the Ta content is preferably 0.02 mass % or more.

-   Hf: 0.5 mass % or less

Hafnium may be contained since it is effective against delay of microstructural changes and prevents degradation of fatigue strength, in particular, rolling fatigue strength. However, Hf does not contribute to further strength improvements even when the Hf content is increased over 0.5 mass %. Thus, the Hf content is 0.5 mass % or less. In order to achieve the effect of improving the fatigue strength, the Hf content is preferably 0.02 mass % or more.

-   Sb: 0.015 mass % or less

Antimony may be contained since it is effective against delay of microstructural changes and prevents degradation of fatigue strength, in particular, rolling fatigue strength. However, when the Sb content is increased to exceed 0.015 mass %, the toughness is decreased. The Sb content is thus 0.015 mass % or less and preferably 0.010 mass % or less. The Sb content is preferably 0.005 mass % or more to allow Sb to exhibit an effect of improving the fatigue strength.

In adding one or more of the above-described seven components to the main components, formula (3) below must be satisfied for the same reason as that for formula (1) above: C^(1/2)(1+0.7Si)(1+3Mn)(1+2.1Cr)(1+3.0 Mo)(1+0.4 Cu)(1+0.3Ni)(1+5.0V)(1+1000B)(1+0.5W)>2.0   (3)

In this invention, Si: 0.1 mass % or less, Pb: 0.1 mass % or less, Bi: 0.1 mass % or less, Se: 0.1 mass % or less, Te: 0.1 mass % or less, Ca: 0.01 mass % or less, Mg: 0.01 mass % or less, and REM: 0.1 mass % or less may be further contained.

-   S: 0.1 mass % or less

Sulfur is a useful element that forms MnS in the steel and improves the machinability. At a S content exceeding 0.1 mass %, however, S segregates in the grain boundaries and thereby decreases the grain boundary strength. Thus, the S content is limited to 0.1 mass %or less and preferably 0.04 mass % or less.

-   Pb: 0.1 mass % or less -   Bi: 0.1 mass % or less

Both Pb and Bi improve machinability since they melt during machining to provide lubricating and embrittlement effects. Thus they may be added for these effects. However, at a Pb content exceeding 0.1 mass % and a Bi content exceeding 0.1 mass %, the effects are saturated and the cost of the components increases. Thus, the Pb and Bi contents are set to be within the above-described ranges. In order to improve the machinability, the Pb content is preferably 0.01 mass % or more and the Bi content is preferably 0.01 mass % or more.

-   Se: 0.1 mass % or less -   Te: 0.1 mass % or less

Se and Te each bond with Mn to form MnSe and MnTe, which function as chip-breakers and thereby improve the machinability. However, at a content exceeding 0.1 mass %, the effects are saturated and the cost for the component increases. Thus, the content is set to be 0.1 mass % or less for both the elements. In order to improve the machinability, the Se content is preferably 0.003 mass % or more and the Te content is preferably 0.003 mass % or more.

-   Ca: 0.01 mass % or less -   REM: 0.1 mass % or less

Ca and REM each bond with MnS to form sulfides, which function as chip-breakers and thereby improve the machinability. However, at a Ca content exceeding 0.01 mass % and a REM content exceeding 0.1 mass %, the effects are saturated, and the cost for the component increases. Thus, their contents are set to be within the above-described ranges. In order to improve the machinability, the Ca content is preferably 0.0001 mass % or more and the REM content is preferably 0.0001 mass % or more.

-   Mg: 0.01 mass % or less

Magnesium is not only an oxidizer but also functions as the stress concentration source thereby improving the machinability. Thus, Mg may added as necessary. However, at an excessively large Mg content, the effects are saturated and the cost for the component increases. Thus, the Mg content is set to be 0.01 mass % or less. In order to improve the machinability the Mg content is preferably 0.0001 mass % or more.

The balance excluding the above-described elements is preferably Fe and inevitable impurities. Examples of the inevitable impurities are P, 0, and N. A P content up to 0.10 mass %, a N content up to 0.01 mass %, and an 0 content of up to 0.008 mass % are acceptable.

Next, the production method of the present invention is explained.

A steel material having a particular composition described above is subjected to bar-mill rolling and then to hot-working such as hot-forging to form a component-shaped product. The component is at least partially subjected to high-frequency hardening at a heating temperature of 800° C. to 1,000° C. This portion of the component subjected to the hardening is set to be the site where fatigue strength is required.

In this series of steps, a hardened structure having an average former austenitic grain diameter of 12 μm or less and a maximum grain diameter not more than four times the average diameter can be obtained by conducting the hot-working such that the total processing ratio in the temperature range of 800° C. to 1,000° C. is at least 80%, cooling at a rate of at least 0.2° C./s in the temperature range of 700° C. to 500° C., and then effecting at least 20% or processing in the temperature range of below 800° C. or by conducting the hot-working such that the total processing ratio in the temperature range of 800° C. to 1,000° C. is at least 80%, effecting 20% or more of processing in the temperature range of below 800° C., and then cooling at a rate of 0.2° C./s or more in the temperature range of 700° C. to 500° C.; and then employing the high-frequency hardening conditions described in detail below.

Each of the limitations will now be described in detail.

[Working Conditions]

The total processing ratio in the range of 800° C. to 1,000° C. during the hot-working is set to be at least 80%, and the cooling rate in the temperature range of 700° C. to 500° C. is set to be at least 0.2° C./s. Under these conditions, the pre-hardening structure can be obtained as homogeneous, fine bainite and/or martensitic structure, and the austenitic grains become finer during the subsequent heating for high-frequency hardening. Preferably, the cooling rate is at least 0.5° C./s.

Before the high-frequency hardening, 20% or more of processing is effected in the temperature range below 800° C. The working in the temperature range of 800° C. or lower may be conducted in the hot-working step and before the cooling at the above-described cooling ratio (the temperature range from 700° C. to less than 800° C.); alternatively, a separate cold working may be effected after the cooling or a hot-working may be effected by re-heating the workpiece at the A₁ transformation temperature or less. The processing ratio at a temperature below 800° C. is preferably at least 30%. Examples of the working methods include cold forging, cold ironing, forming rolling, and shotblasting. By working at a temperature of 800° C. or less, the bainite or martensitic structure before the high-frequency hardening becomes finer; therefore, the average former austenitic grain diameter in the hardened layer obtained by the high-frequency hardening will be 12 μm or less and the maximum grain diameter does not exceed four times the average grain diameter. As a result, the fatigue strength is improved.

The term “processing ratio” used herein is defined as the reduction ratio of the cross-sectional area before and after working for the cases concerning rolling, forging, and drawing. For the cases such as shotblasting where the processing ratio cannot be defined in terms of the reduction-ratio of the cross-sectional area, the processing ratio is determined by the change in hardness corresponding to the reduction ratio.

[High-Frequency Hardening Conditions]

The heating temperature is set to be 800° C. to 1,000° C., and the range 600° C. to 800° C. is heated at a rate of 400° C./s or more. If the heating temperature is less than 800° C., generation of the austenitic structure is insufficient, and a hardened layer cannot be obtained. On the other hand, when the heating temperature exceeds 1,000° C., the rate of austenitic grain growth increases substantially. This increases the average grain diameter and, at the same time, the individual grains frequently grow at rates significantly different from one another in the temperature region where rapid growth occurs. As a result, the maximum grain diameter exceeds four times the average grain diameter, thereby inflicting a decrease in fatigue strength.

When the heating ratio in the range of 600° C. to 800° C. is less than 400° C./s, the austenitic grain growth is promoted and the size of the individual grains becomes nonuniform. Thus, the maximum grain diameter exceeds four times the average grain diameter, which results in a decreased fatigue strength. This is presumably due to the fact that when the heating rate is low, reverse transformation from ferrite to austenite starts at a lower temperature and nonuniform grain growth tends to occur depending on the position of the grains.

The heating temperature is preferably 800° C. to 950° C., and the heating rate in the range of 600° C. to 800° C. is preferably at least 700° C./s and more preferably at least 1,000° C./s.

During the high-frequency heating, if the retention time at 800° C. or more is excessively long, the austenitic grains grow and the maximum grain diameter tends to exceed four times the average grain diameter as a result. Thus, the retention time at 800° C. or more is preferably 5 sec or less.

EXAMPLES Example 1

As an example of the component for machine structural use, shafts simulating a drive shaft, an output shaft, and an input shaft of automobiles were prepared. In particular, each of the steel materials having compositions shown in Table 3 was melted in a converter and cast into a cast slab by continuous casting. The size of the cast slab was 300×400 [mm]. The cast slab was subjected to a breakdown step and rolled into a 150 mm square billet. The billet was rolled into a steel bar under the hot-working conditions shown in Tables 4-1 and 4-2 while setting the finishing temperature to 800° C. or more. Here, the total processing ratio in the range of 800° C. to 1,000° C. is the ratio of reduction of the cross section in this temperature range. After the rolling, cooling was conducted under the conditions described in Tables 4-1 and 4-2.

Next, the steel bar was cut to a predetermined length and subjected to surface cutting and partly cold-drawing to adjust the diameter. Simultaneously, splines of the steel bar are formed by rolling to prepare a shaft 1 having splines 2 with dimensions and a shape shown in FIG. 1. The cold-working ratio is the ratio of reduction of the cross-section.

This shaft was hardened under conditions shown in Tables 4-1 and 4-2 using a high-frequency hardening apparatus with a frequency of 15 kHz and then tempered at 170° C. for 30 minutes in a heating furnace. The torsional fatigue strength was tested. For some of the shafts, tempering was omitted and their torsional fatigue strength was tested.

The torsional fatigue strength was evaluated in terms of torque (N·m) when the number of cycles to fracture was 1×10⁵ in the shaft torsional fatigue test. In the torsional fatigue test, a hydraulic fatigue tester was used, and as shown in FIG. 2, splines 2 a and 2 b were respectively fit into disk-shaped chucks 3 a and 3 b and torsional torque was repeatedly applied between the chucks 3 a and 3 b at a frequency of 1 to 2 Hz.

The hardened layer of the same shaft was etched using an etchant whose main component is picric acid, e.g., a mixture of an aqueous picric acid solution prepared by dissolving 50 g of picric acid in 500 g of water, 11 g of sodium dodecylbenzenesulfonate, 1 g of ferrous chloride, and 1.5 g of oxalic acid. Subsequently, its structure was observed with an optical microscope to determine the average diameter and maximum diameter of the former austenitic grains. The average diameter and the maximum diameter were determined by the same method as one described above.

The resistance to quenching crack was also tested for the same shaft.

The resistance to quenching crack was evaluated in terms of the number of quenching cracks observed, with an optical microscope (magnification: ×100 to ×200), in five polished C-cross-sections of the splines after the high-frequency hardening.

The results are shown in Tables 4-1 and 4-2. TABLE 3 Chemical composition (mass %) Value of left Steel type C Si Mn P S Al N Others part of formula (1)-(3) A 0.48 0.55 0.78 0.011 0.019 0.024 0.0043 — 3.2 B 0.45 0.51 0.81 0.013 0.022 0.029 0.0041 0.3Mo, 0.4Ni 6.6 C 0.43 0.51 0.79 0.011 0.021 0.024 0.0039 0.45Mo, 0.021Ti, 0.0024B 22.6 D 0.43 0.21 0.91 0.011 0.022 0.025 0.0041 0.9Cr, 0.003Ca 8.1 E 0.25 0.21 0.92 0.011 0.022 0.024 0.0042 — 2.2 F 1.52 0.19 0.45 0.012 0.015 0.025 0.0042 1.32Cr 11.5 G 0.45 0.22 2.55 0.011 0.018 0.025 0.0045 — 6.7 H 0.32 0.21 0.55 0.012 0.015 0.025 0.0045 — 1.7 I 0.43 0.21 0.91 0.011 0.022 0.025 0.0041 1.1Cr 9.3 J 0.45 0.21 0.92 0.011 0.022 0.026 0.0041 0.4Mo 6.4 K 0.43 0.22 0.91 0.011 0.022 0.025 0.0042 0.8Cu 3.7 L 0.43 0.21 0.91 0.011 0.022 0.025 0.0041 0.8Ni 3.5 M 0.43 0.21 0.91 0.011 0.022 0.023 0.0043 0.15V 4.9 N 0.44 0.23 0.92 0.011 0.022 0.025 0.0041 0.7W 3.9 O 0.43 0.21 0.91 0.012 0.022 0.025 0.0039 0.06Ti 2.8 P 0.44 0.21 0.91 0.011 0.019 0.025 0.0041 0.03Nb 2.8 Q 0.43 0.21 0.91 0.011 0.022 0.024 0.0042 0.05Zr 2.8 R 0.43 0.21 0.91 0.011 0.022 0.024 0.0042 0.003B 2.8 S 0.48 0.55 0.78 0.011 0.004 0.024 0.0043 — 3.2 T 0.48 0.55 0.78 0.011 0.004 0.024 0.0043 0.02Pb 3.2 U 0.48 0.55 0.78 0.011 0.004 0.024 0.0043 0.02Pb 3.2 V 0.48 0.55 0.78 0.011 0.004 0.024 0.0043 0.01Se 3.2 W 0.48 0.55 0.78 0.011 0.004 0.024 0.0043 0.01Te 3.2 X 0.48 0.55 0.78 0.011 0.004 0.024 0.0043 0.003Ca 3.2 Y 0.48 0.55 0.78 0.011 0.022 0.024 0.0043 0.002Mg 3.2 Z 0.48 0.55 0.78 0.011 0.022 0.024 0.0043 0.002REM 3.2 AA 0.45 0.21 0.92 0.012 0.022 0.026 0.0041 0.4Mo, 0.02Ti 6.4 AB 0.45 0.22 0.92 0.011 0.022 0.026 0.0041 0.02Ti, 0.003B 11.6 AC 0.45 0.21 0.92 0.011 0.004 0.026 0.0041 0.4Mo 6.4 AD 0.43 0.21 0.91 0.012 0.004 0.025 0.0039 0.06Ti 2.8 AE 0.45 0.21 0.92 0.011 0.004 0.026 0.0041 0.4Mo, 0.02Ti 6.4 AF 0.46 0.23 0.92 0.011 0.022 0.026 0.0041 0.02Pb 3.0 AG 0.45 0.21 0.92 0.011 0.019 0.026 0.0041 0.4Mo, 0.02Pb 6.4 AH 0.46 0.21 0.91 0.011 0.019 0.025 0.0041 0.03Nb, 0.02Pb 2.9 AI 0.45 0.21 0.92 0.011 0.019 0.026 0.0041 0.4Mo, 0.02Ti, 0.02Pb 6.4 AJ 0.45 0.45 0.82 0.011 0.031 0.002 0.004 0.4Mo, 0.02Ti, 0.0020B, 0.003Ca 26.9 AK 0.43 0.51 0.79 0.011 0.06 0.002 0.0039 0.45Mo, 0.021Ti, 0.0020B, 0.003Ca 22.6 AL 0.43 0.51 0.79 0.011 0.06 <0.001 0.0039 0.45Mo, 0.021Ti, 0.0020B, 0.003Ca 22.6

TABLE 4-1 Pre-hardening Hot-working conditions Working (cold- structure HF hardening conditions Total processing Cooling rate working) ratio Proportion Heating Retention time Steel ratio (%) at (° C./s) after (%) at less than (vol. %) Heating temperature (sec) at 800° C. No. type 800° C. to 1000° C. working 800° C. of bainite structure rate (° C./s) (° C.) or more  1 A 30 0.7 20 73 800 880 1  2 A 80 0.8 20 71 800 890 1  3 A 80 0.7 50 72 800 870 1  4 A 80 0.06 20 4 800 910 1  5 A 80 0.8 0 72 800 890 1  6 B 80 0.8 20 75 800 870 1  7 C 30 0.7 20 84 800 880 1  8 C 80 0.8 0 83 800 880 1  9 C 80 0.7 20 85 800 880 1 10 C 80 0.8 50 82 800 880 1 11 C 80 0.08 20 7 800 880 1 12 C 80 0.8 0 86 3000 880 1 13 C 80 0.8 10 84 200 900 1 14 C 80 0.8 20 85 800 700 0 15 C 80 0.8 20 86 800 1050 1 16 D 80 0.8 20 79 800 890 1 17 E 80 0.7 20 9 800 890 1 18 F 80 0.7 20 75 800 890 1 19 G 80 0.7 20 70 800 890 1 20 H 80 0.7 20 10 800 890 1 21 A 80 0.8 20 75 2000 800 1 22 I 80 0.8 20 68 800 890 1 23 J 80 0.8 20 82 800 890 1 Torsional Hardened layer structure fatigue Average former Maximum former Number (N) of strength austenitic grain austenitic grain quenching (1 × 10⁵) No. diameter (μm) diameter (μm) cracks Tempering (MPa) Reference  1 6.5 35 0 Yes 559 Comp. Ex.  2 4.8   9.5 0 Yes 845 Inv. Ex.  3 2.5   5.3 0 Yes 891 Inv. Ex.  4 13.5  40 5 Yes 560 Comp. Ex.  5 7.5 48 0 Yes 570 Comp. Ex.  6 4.8 11 0 Yes 888 Inv. Ex.  7 5.6 30 0 Yes 680 Comp. Ex.  8 4.8 35 0 Yes 670 Comp. Ex.  9 2.5  7 0 Yes 880 Inv. Ex. 10 1.7  5 0 Yes 925 Inv. Ex. 11 6.5 35 0 Yes 670 Comp. Ex. 12 3.5 20 2 Yes 620 Comp. Ex. 13 8.9 45 0 Yes 622 Comp. Ex. 14 Undetectable Undetectable 0 Yes 320 Comp. Ex. due to due to incomplete incomplete hardening hardening 15 15.5  50 0 Yes 610 Comp. Ex. 16 4.5 15 0 Yes 889 Inv. Ex. 17 7.5 45 5 Yes 542 Comp. Ex. 18 4.5 35 3 Yes 615 Comp. Ex. 19 4.8 32 0 Yes 720 Comp. Ex. 20 4.2 35 2 Yes 535 Comp. Ex. 21 2.8  9 0 Yes 790 Inv. Ex. 22 4.2 12 0 Yes 845 Inv. Ex. 23 4.3 12 0 Yes 847 Inv. Ex. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

TABLE 4-2 Hot-working conditions Working (cold- Total processing working) ratio (%) ratio (%) at at less than Pre- HF hardening conditions 800° C. to 1000° C. 800° C. hardening Retention Cooling rate Proportion (vol. %) structure Heating time (sec) at Average former Steel (° C./s) after of bainite Heating temperature 800° C. or austenitic grain No. type working structure(vol. %) rate(° C./s) (° C.) more diameter (μm) 24 K 80 0.8 20 79 800 890 1 25 L 80 0.8 20 79 800 890 1 26 M 80 0.8 20 79 800 890 1 27 N 80 0.8 20 79 800 890 1 28 O 80 0.8 20 72 800 890 1 29 p 80 0.8 20 79 800 890 1 30 Q 80 0.8 20 79 800 890 1 31 R 80 0.8 20 79 800 890 1 32 S 80 0.8 20 70 800 890 1 33 T 80 0.8 20 79 800 890 1 34 U 80 0.8 20 79 800 890 1 35 V 80 0.8 20 79 800 890 1 36 W 80 0.8 20 79 800 890 1 37 X 80 0.8 20 79 800 890 1 38 Y 80 0.8 20 79 800 890 1 39 Z 80 0.8 20 79 800 890 1 40 AA 80 0.8 20 85 800 890 1 41 AB 80 0.8 20 79 800 890 1 42 AC 80 0.8 20 80 800 890 1 43 AD 80 0.8 20 79 800 890 1 44 AE 80 0.8 20 79 800 890 1 45 AF 80 0.8 20 79 800 890 1 46 AG 80 0.8 20 79 800 890 1 47 AH 80 0.8 20 79 800 890 1 48 AI 80 0.8 20 85 800 890 1 49 C 80 0.8 50 82 800 880 1 50 AJ 80 0.8 20 83 800 930 2 51 AK 80 0.8 20 81 500 930 3 52 AL 80 0.8 20 81 500 930 3 Hardened Torsional layer structure Number (N) fatigue Maximum former of strength austenitic grain quenching (1 × 10⁵) No. diameter (μm) cracks Tempering (MPa) Reference 24 3.9 11 0 Yes 849 Inv. Ex. 25 4.3 12 0 Yes 851 Inv. Ex. 26 4.2 12 0 Yes 852 Inv. Ex. 27 4.3 12 0 Yes 855 Inv. Ex. 28 3.9 11 0 Yes 858 Inv. Ex. 29 4.3 12 0 Yes 861 Inv. Ex. 30 3.8 10 0 Yes 864 Inv. Ex. 31 4.3 12 0 Yes 867 Inv. Ex. 32 4.2 12 0 Yes 846 Inv. Ex. 33 4.3 12 0 Yes 873 Inv. Ex. 34 4.5 13 0 Yes 845 Inv. Ex. 35 4.3 12 0 Yes 865 Inv. Ex. 36 3.8 10 0 Yes 889 Inv. Ex. 37 4.3 12 0 Yes 855 Inv. Ex. 38 4.2 12 0 Yes 858 Inv. Ex. 39 4.3 12 0 Yes 861 Inv. Ex. 40 3.9 11 0 Yes 864 Inv. Ex. 41 4.3 12 0 Yes 867 Inv. Ex. 42 3.8 10 0 Yes 846 Inv. Ex. 43 4.3 12 0 Yes 873 Inv. Ex. 44 3.9 11 0 Yes 855 Inv. Ex. 45 4.3 12 0 Yes 858 Inv. Ex. 46 3.8 10 0 Yes 861 Inv. Ex. 47 4.2 12 0 Yes 864 Inv. Ex. 48 3.7 10 0 Yes 867 Inv. Ex. 49 1.7  5 0 No 1000  Inv. Ex. 50 3.1 10 0 No 900 Inv. Ex. 51 3.9 10 0 No 910 Inv. Ex. 52 4.0 10 0 Yes 905 Inv. Ex. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

Tables 4-1 and 4-2 clearly show that every shaft with a hardened structure in which the average former austenitic grain diameter was 12 μm or less and the maximum grain diameter did not exceed four times the average grain diameter exhibited high torsional fatigue strength and excellent resistance to quench cracking, i.e., the number of quench cracking being zero.

In contrast, shafts with hardened structures in which the average former austenitic grain diameter was 12 μm or more and the maximum grain diameter exceeded four times the average grain diameter all exhibited low fatigue strength.

Example 2

A crankshaft shown in FIG. 8 was prepared as a component for machine structural use of the present invention. A crankshaft 4 has journals 5 to a cylinder, crank pins 6, which are bearings for a piston connecting rod, crank webs 7, and counter weights 8. The journals 5 and the crank pins 6 are subjected to high-frequency hardening to increase the fatigue strength.

Each of steel materials having compositions shown in Table 3 was melted in a converter and cast into a cast slab by continuous casting. The size of the cast slab was 300×400 [mm]. The cast slab was hot-rolled to prepare a steel bar having a diameter of 90 mm. The steel bar was cut to a predetermined length, hot-forged in the temperature range of 700° C. to 1,100° C. from bending to finish, deburred, and formed into a shape of a crankshaft, followed by cooling at a rate shown in Tables 5-1 and 5-2. During the hot-forging, the hot-forging process was divided into a plurality of steps. The last stage of the hot-forging was conducted at 700° C. to less than 800° C., and all other steps were conducted at 800° C. to 1,000° C. In each of the steps, the diameter of the crank pins was adjusted to control the total processing ratio at 800° C. to 1,000° C. and the total processing ratio at 700° C. to less than 800° C.

Subsequently, as shown in a cross-sectional view of the crankshaft in FIG. 9, the surfaces of the crank pins and journals of the crankshaft were subjected to high-frequency hardening under the conditions set forth in Tables 5-1 and 5-2 to form hardened structure layers 9. The crankshaft was then tempered at 170° C. for 30 min and subjected to finishing to prepare a product. Some of the crankshafts were not subjected to tempering.

The bending fatigue life of the crankshaft obtained thereby was tested. The results are shown in Tables 5-1 and 5-2.

The bending fatigue life of the crankshaft was evaluated as follows.

As shown in FIG. 10, while fixing the ends of the crankshaft, an endurance test was conducted by repeatedly applying a predetermined load (5,000 N) to each connecting rod. The number of cycles of load application before breaking of the pin or journal was used for evaluating the bending fatigue life.

The average diameter and the maximum grain diameter of the former austenitic grains in the hardened layer of the same crankshaft were determined by the same method as one described above.

The results are also described in Tables 5-1 and 5-2. TABLE 5-1 Hot-working conditions Pre-hardening HF hardening conditions Total processing Cooling rate Total processing structure Heating Retention time Steel ratio (%) at (° C./s) after ratio (%) at less Proportion (vol. %) Heating temperature (sec) at 800° C. No. type 800° C. to 1000° C. working than 800° C. of bainite structure rate (° C./s) (° C.) or more  1 A 30 0.7 20 73 800 880 1  2 A 80 0.8 20 71 800 890 1  3 A 80 0.7 50 72 800 870 1  4 A 80 0.06 20 4 800 910 1  5 A 80 0.8 0 72 800 890 1  6 B 80 0.8 20 75 800 870 1  7 C 30 0.7 20 84 800 880 1  8 C 80 0.8 0 83 800 880 1  9 C 80 0.7 20 85 800 880 1 10 C 80 0.8 50 82 800 880 1 11 C 80 0.08 20 7 800 880 1 12 C 80 0.8 0 86 3000 880 1 13 C 80 0.8 10 84 200 900 1 14 C 80 0.8 20 85 800 700 0 15 C 80 0.8 20 86 800 1050 1 16 D 80 0.8 20 79 800 890 1 17 E 80 0.7 20 9 800 890 1 18 F 80 0.7 20 75 800 890 1 19 G 80 0.7 20 70 800 890 1 20 H 80 0.7 20 10 800 890 1 21 A 80 0.8 20 75 2000 800 1 22 I 80 0.8 20 68 800 890 1 23 J 80 0.8 20 82 800 890 1 Hardened layer structure Average former Maximum former Bending austenitic grain austenitic grain fatigue life No. diameter (μm) diameter (μm) Tempering (×10⁶) Reference  1 6.5 35 Yes 2.2 Comp. Ex.  2 4.8   9.5 Yes 9.5 Inv. Ex.  3 2.5   5.3 Yes 12.0 Inv. Ex.  4 13.5  40 Yes 2.2 Comp. Ex.  5 7.5 48 Yes 2.3 Comp. Ex.  6 4.8 11 Yes 11.8 Inv. Ex.  7 5.6 30 Yes 4.1 Comp. Ex.  8 4.8 35 Yes 3.9 Comp. Ex.  9 2.5  7 Yes 11.4 Inv. Ex. 10 1.7  5 Yes 14.3 Inv. Ex. 11 6.5 35 Yes 3.9 Comp. Ex. 12 3.5 20 Yes 3.0 Comp. Ex. 13 8.9 45 Yes 3.1 Comp. Ex. 14 Undetectable Undetectable Yes 0.7 Comp. Ex. due to due to incomplete incomplete hardening hardening 15 15.5  50 Yes 2.9 Comp. Ex. 16 4.5 15 Yes 11.9 Inv. Ex. 17 7.5 45 Yes 2.0 Comp. Ex. 18 4.5 35 Yes 2.9 Comp. Ex. 19 4.8 32 Yes 5.0 Comp. Ex. 20 4.2 35 Yes 2.0 Comp. Ex. 21 2.8  9 Yes 9.4 Inv. Ex. 22 4.2 12 Yes 9.5 Inv. Ex. 23 4.3 12 Yes 9.6 Inv. Ex. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

TABLE 5-2 Hot-working conditions Pre-hardening HF hardening conditions Total processing Cooling rate Total processing structure Heating Retention time Steel ratio (%) at (° C./s) after ratio (%) at less Proportion (vol. %) Heating temperature (sec) at 800° C. No. type 800° C. to 1000° C. working than 800° C. of bainite structure rate (° C./s) (° C.) or more 24 K 80 0.8 20 79 800 890 1 25 L 80 0.8 20 79 800 890 1 26 M 80 0.8 20 79 800 890 1 27 N 80 0.8 20 79 800 890 1 28 O 80 0.8 20 72 800 890 1 29 P 80 0.8 20 79 800 890 1 30 Q 80 0.8 20 79 800 890 1 31 R 80 0.8 20 79 800 890 1 32 S 80 0.8 20 70 800 890 1 33 T 80 0.8 20 79 800 890 1 34 U 80 0.8 20 79 800 890 1 35 V 80 0.8 20 79 800 890 1 36 W 80 0.8 20 79 800 890 1 37 X 80 0.8 20 79 800 890 1 38 Y 80 0.8 20 79 800 890 1 39 Z 80 0.8 20 79 800 890 1 40 AA 80 0.8 20 85 800 890 1 41 AB 80 0.8 20 79 800 890 1 42 AC 80 0.8 20 80 800 890 1 43 AD 80 0.8 20 79 800 890 1 44 AE 80 0.8 20 79 800 890 1 45 AF 80 0.8 20 79 800 890 1 46 AG 80 0.8 20 79 800 890 1 47 AH 80 0.8 20 79 800 890 1 48 AI 80 0.8 20 85 800 890 1 49 C 80 0.8 50 82 800 880 1 50 AJ 80 0.8 20 83 800 930 2 51 AK 80 0.8 20 81 500 930 3 52 AK 80 0.8 20 81 500 930 3 Hardened layer structure Average former Maximum former Bending austenitic grain austenitic grain fatigue life No. diameter (μm) diameter (μm) Tempering (×10⁶) Reference 24 3.9 11 Yes 9.7 Inv. Ex. 25 4.3 12 Yes 9.8 Inv. Ex. 26 4.2 12 Yes 9.9 Inv. Ex. 27 4.3 12 Yes 10.0 Inv. Ex. 28 3.9 11 Yes 10.2 Inv. Ex. 29 4.3 12 Yes 10.3 Inv. Ex. 30 3.8 10 Yes 10.5 Inv. Ex. 31 4.3 12 Yes 10.6 Inv. Ex. 32 4.2 12 Yes 9.6 Inv. Ex. 33 4.3 12 Yes 11.0 Inv. Ex. 34 4.5 13 Yes 9.5 Inv. Ex. 35 4.3 12 Yes 10.5 Inv. Ex. 36 3.8 10 Yes 11.9 Inv. Ex. 37 4.3 12 Yes 10.0 Inv. Ex. 38 4.2 12 Yes 10.2 Inv. Ex. 39 4.3 12 Yes 10.3 Inv. Ex. 40 3.9 11 Yes 10.5 Inv. Ex. 41 4.3 12 Yes 10.6 Inv. Ex. 42 3.8 10 Yes 9.6 Inv. Ex. 43 4.3 12 Yes 11.0 Inv. Ex. 44 3.9 11 Yes 10.0 Inv. Ex. 45 4.3 12 Yes 10.2 Inv. Ex. 46 3.8 10 Yes 10.3 Inv. Ex. 47 4.2 12 Yes 10.5 Inv. Ex. 48 3.7 10 Yes 10.6 Inv. Ex. 49 1.7  5 No 15.2 Inv. Ex. 50 3.1 10 Yes 10.5 Inv. Ex. 51 3.9 10 Yes 10.5 Inv. Ex. 52 4.0 10 Yes 10.5 Inv. Ex. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

Tables 5-1 and 5-2 clearly show that every shaft with a hardened structure where an average grain diameter of former austenitic grains in the hardened layer was 12 μm or less and a maximum grain diameter did not exceed four times the average diameter exhibited excellent bending fatigue life, i.e., the number of times of application of the load to breaking being 9×10⁶ or more.

In contrast, those having an average grain diameter of former austenitic grains of 12 μm or less and a maximum grain diameter exceeding four times the average grain diameter exhibited low bending life strength.

Example 3

A constant velocity joint 12 for transmitting power from a drive shaft 10 to a hub 11 of a wheel was prepared as a component for machine structural use of the present invention.

The constant velocity joint 12 is a combination of an outer race 13 and an inner race 14. In particular, the inner race 14 is movably fixed to the inner side of a mouth 13 a of the outer race 13 via balls 15 fit in a ball track groove in the inner surface of the mouth 13 a and is connected to the drive shaft 10; meanwhile, a stem 13 b of the outer race 13 is for example splined to the hub 11 to transmit power from the drive shaft 10 to the hub 11 of the wheel.

Each of steel materials having compositions shown in Table 3 was melted in a converter and cast into a cast slab by continuous casting. The size of the cast slab was 300×400 [mm]. The cast slab was subjected to a breakdown step and rolled into a 150 mm square billet. The billet was then rolled to prepare a steel bar having a diameter of 50 mm.

Next, the steel bar was cut to a predetermined length, hot-forged under the conditions set forth in Table 6-1, 6-2, 7-1, or 7-2 at a temperature 800° C. or more, and formed such that the mouth (outer diameter: 600 mm) and the stem (diameter: 20 mm) of the constant velocity joint outer race were integrally combined. Subsequently, the tracking groove for balls is formed in the inner surface of the mouth of the constant velocity joint outer race by cutting or cold forging, and, at the same time, the stem of the constant velocity joint outer race was formed into a spline shaft by cutting or rolling. The cooling after the hot-forging were conducted under the conditions set forth in Tables 6-1, 6-2, 7-1, and 7-2. The total processing ratio in the hot-forging and the forming by rolling was adjusted by controlling the rate of change in cross-sectional area of a cross section taken in the direction orthogonal to the axis direction of the component to which the high-frequency hardening was effected.

As shown in FIG. 12 or 13, the inner surface of the mouth 13 a or the outer surface of the stem 13 b of the constant velocity joint outer race 13 was hardened using a 15 kHz high-frequency hardening apparatus to form a hardened structure layer 16 and then tempered at 180° C. for 2 hours in a heating furnace to prepare a product. The conditions for the tempering were set forth in Tables 6-1, 6-2, 7-1, and 7-2. The tempering was omitted for some of the constant velocity joint outer races. The resulting constant velocity joint outer race was attached, via balls (steel balls) in the mouth, to the inner race connected to the drive shaft, and the stem was fit into the hub to prepare a constant velocity joint unit (see FIG. 11). The specifications for the balls, the inner race, and the hub were as follows:

balls: hardened and tempered high-carbon chromium bearing steel SUJ2

inner race: carburized, hardened, and tempered chromium steel SCr

hub: carburized steel for machine structural use

Next, using this constant velocity joint unit, the endurance test for determining rolling fatigue strength was conducted for those units in which the inner surface of the mouth was subjected to high-frequency hardening and the endurance test for determining torsional fatigue strength was conducted for those units in which the outer surface of the stem was subjected to high-frequency hardening, both the tests being conducted in a power transmission system for transmitting the rotary action of the drive shaft to the hub through the inner race and the outer race of the constant velocity joint.

In the rolling fatigue strength test, power transmission was conducted at a torque of 900 N·m, a working angle (the angle defined by the axis line of the outer race and the axis line of the drive shaft) of 20°, and a number of revolution of 300 rpm. The time taken until the inner surface of the mouth was broken by rolling fatigue was evaluated as the rolling fatigue strength.

In this power transmission system, the endurance test for determining the torsional fatigue strength was conducted. In the torsional fatigue test, the working angle of the constant velocity joint unit (the angle defined by the axis line of the outer race and the axis line of the drive shaft) was set to 0° and a torsional fatigue tester with a maximum torque of 4900 N·m was used to apply torsional force between the hub and the drive shaft, and the maximum torque of the stem was changed so that the stress conditions are fully reversed. The stress at which the number of cycles to fracture was 1×10⁵ was evaluated as the torsional fatigue strength.

In the torsional fatigue test, the shape and dimensions of the hub and the drive shaft were adjusted so that the strength of the hub and the drive shaft were large enough for the evaluation of the torsional fatigue of the outer race of the constant velocity joint.

Likewise, in the rolling fatigue test, the shape and dimensions of the constant velocity joint inner race, steel balls, and the like were adjusted so that the inner surface of the constant velocity joint outer race was the weakest part in the endurance test.

The constant velocity joint outer races prepared under the same conditions were analyzed to determine the average diameter and the maximum diameter of the former austenitic grains in the hardened layer by the above-described process.

The results are shown in Tables 6-1, 6-2, 7-1, and 7-2. TABLE 6-1 Hot-working conditions Total Pre-hardening HF hardening conditions processing ratio Cooling rate Total processing structure Heating Retention time Steel (%) at 800° C. to (° C./s) after ratio (%) at less Proportion (vol. %) Heating temperature (sec) at 800° C. No. type 1000° C. working than 800° C. *1 of bainite structure rate ° C./s) (° C.) or more  1 A 30 0.7 20 73 800 880 1  2 A 80 0.8 20 71 800 890 1  3 A 80 0.7 50 72 800 870 1  4 A 80 0.06 20 4 800 910 1  5 A 80 0.8 0 72 800 890 1  6 B 80 0.8 20 75 800 870 1  7 C 30 0.7 20 84 800 880 1  8 C 80 0.8 0 83 800 880 1  9 C 80 0.7 20 85 800 880 1 10 C 80 0.8 50 82 800 880 1 11 C 80 0.08 20 7 800 880 1 12 C 80 0.8 0 86 3000 880 1 13 C 80 0.8 10 84 200 900 1 14 C 80 0.8 20 85 800 700 0 15 C 80 0.8 20 86 800 1050 1 16 D 80 0.8 20 79 800 890 1 17 E 80 0.7 20 9 800 890 1 18 F 80 0.7 20 75 800 890 1 19 G 80 0.7 20 70 800 890 1 20 H 80 0.7 20 10 800 890 1 21 A 80 0.8 20 75 2000 800 1 22 I 80 0.8 20 68 800 890 1 23 J 80 0.8 20 82 800 890 1 24 K 80 0.8 20 79 800 890 1 25 L 80 0.8 20 79 800 890 1 26 M 80 0.8 20 79 800 890 1 27 N 80 0.8 20 79 800 890 1 Hardened layer structure Average former Maximum former austenitic grain austenitic grain Rolling fatigue No. diameter (μm) diameter (μm) Tempering life (hr) Reference  1 6.5 35 Yes 79 Comp. Ex.  2 4.8   9.5 Yes 136 Inv. Ex.  3 2.5   5.3 Yes 148 Inv. Ex.  4 13.5  40 Yes 79 Comp. Ex.  5 7.5 48 Yes 81 Comp. Ex.  6 4.8 11 Yes 147 Inv. Ex.  7 5.6 30 Yes 99 Comp. Ex.  8 4.8 35 Yes 97 Comp. Ex.  9 2.5  7 Yes 145 Inv. Ex. 10 1.7  5 Yes 158 Inv. Ex. 11 6.5 35 Yes 97 Comp. Ex. 12 3.5 20 Yes 89 Comp. Ex. 13 8.9 45 Yes 89 Comp. Ex. 14 Undetectable Undetectable Yes 50 Comp. Ex. due to due to incomplete incomplete hardening hardening 15 15.5  50 Yes 87 Comp. Ex. 16 4.5 15 Yes 148 Inv. Ex. 17 7.5 45 Yes 76 Comp. Ex. 18 4.5 35 Yes 88 Comp. Ex. 19 4.8 32 Yes 98 Comp. Ex. 20 4.2 35 Yes 75 Comp. Ex. 21 2.8  9 Yes 139 Inv. Ex. 22 4.2 12 Yes 136 Inv. Ex. 23 4.3 12 Yes 136 Inv. Ex. 24 3.9 11 Yes 137 Inv. Ex. 25 4.3 12 Yes 137 Inv. Ex. 26 4.2 12 Yes 138 Inv. Ex. 27 4.3 12 Yes 139 Inv. Ex. *1: If value in the column is zero, the inner surface of the mouth is formed by cutting. Otherwise, the inner surface of the mouth is formed by cold-forging and the figure in the column indicates the processing ratio. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

TABLE 6-2 Hot-working Hardened conditions Total Pre- layer structure Total processing hardening HF hardening conditions Average Maximum processing Cooling ratio (%) structure Retention former former ratio (%) rate at less Proportion time austenitic austenitic at (° C./s) than (vol. %) Heating Heating (sec) at grain grain Rolling Steel 800° C. to after 800° C. of bainite rate temperature 800° C. diameter diameter Tem- fatigue No. type 1000° C. working *1 structure (° C./s) (° C.) or more (μm) (μm) pering life (hr) Reference 28 O 80 0.8 20 72 800 890 1 3.9 11 Yes 139 Inv. Ex. 28 O 80 0.8 20 72 800 890 1 3.9 11 Yes 139 Inv. Ex. 29 P 80 0.8 20 79 800 890 1 4.3 12 Yes 140 Inv. Ex. 30 Q 80 0.8 20 79 800 890 1 3.8 10 Yes 141 Inv. Ex. 31 R 80 0.8 20 79 800 890 1 4.3 12 Yes 142 Inv. Ex. 32 S 80 0.8 20 70 800 890 1 4.2 12 Yes 136 Inv. Ex. AJ 0.45 0.45 0.82 0.011 0.031 0.002 0.004 0.4Mo, 26.9 12 Yes 143 Inv. Ex. 0.02Ti, 0.0020B, 0.003Ca AK 0.43 0.51 0.79 0.011 0.06 0.002 0.0039 0.45Mo, 22.6 13 Yes 136 Inv. Ex. 0.021Ti, 0.0020B, 0.003Ca 35 V 80 0.8 20 79 800 890 1 4.3 12 Yes 141 Inv. Ex. 36 W 80 0.8 20 79 800 890 1 3.8 10 Yes 148 Inv. Ex. 37 X 80 0.8 20 79 800 890 1 4.3 12 Yes 139 Inv. Ex. 38 Y 80 0.8 20 79 800 890 1 4.2 12 Yes 139 Inv. Ex. 39 Z 80 0.8 20 79 800 890 1 4.3 12 Yes 140 Inv. Ex. 40 AA 80 0.8 20 85 800 890 1 3.9 11 Yes 141 Inv. Ex. 41 AB 80 0.8 20 79 800 890 1 4.3 12 Yes 142 Inv. Ex. 42 AC 80 0.8 20 80 800 890 1 3.8 10 Yes 136 Inv. Ex. 43 AD 80 0.8 20 79 800 890 1 4.3 12 Yes 143 Inv. Ex. 44 AE 80 0.8 20 79 800 890 1 3.9 11 Yes 139 Inv. Ex. 45 AF 80 0.8 20 79 800 890 1 4.3 12 Yes 139 Inv. Ex. 46 AG 80 0.8 20 79 800 890 1 3.8 10 Yes 140 Inv. Ex. 47 AH 80 0.8 20 79 800 890 1 4.2 12 Yes 141 Inv. Ex. 48 AI 80 0.8 20 85 800 890 1 3.7 10 Yes 142 Inv. Ex. 49 C 80 0.8 50 82 800 880 1 1.7 5 No 172 Inv. Ex. 50 AJ 80 0.8 20 83 800 930 2 3.1 10 Yes 140 Inv. Ex. 51 AK 80 0.8 20 81 500 930 3 3.9 10 Yes 138 Inv. Ex. 52 AL 80 0.8 20 81 500 930 3 4.0 10 Yes 135 Inv. Ex. *1: If value in the column is zero, the inner surface of the mouth is formed by cutting. Otherwise, the inner surface of the mouth is formed by cold-forging and the figure in the column indicates the processing ratio. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

TABLE 7-1 Hot-working Hardened conditions Total Pre- layer structure Total processing hardening HF hardening conditions Average Maximum processing Cooling ratio (%) structure Retention former former Torsional ratio (%) rate at less Proportion Heating time austenitic austenitic fatigue at (° C./s) than (vol. %) Heating temper- (sec) at grain grain strength Steel 800° C. to after 800° C. of bainite rate ature 800° C. diameter diameter Tem- (×10⁵) No. type 1000° C. working *2 structure (° C./s) (° C.) or more (μm) (μm) pering (MPa) Reference 1 A 30 0.7 20 73 800 880 1 6.5 35 Yes 481 Comp. Ex. 2 A 80 0.8 20 71 800 890 1 4.8   9.5 Yes 783 Inv. Ex. 3 A 80 0.7 50 72 800 870 1 2.5   5.3 Yes 832 Inv. Ex. 4 A 80 0.06 20 4 800 910 1 13.5  40 Yes 482 Comp. Ex. 5 A 80 0.8 0 72 800 890 1 7.5 48 Yes 493 Comp. Ex. 6 B 80 0.8 20 75 800 870 1 4.8 11 Yes 829 Inv. Ex. 7 C 30 0.7 20 84 800 880 1 5.6 30 Yes 609 Comp. Ex. 8 C 80 0.8 0 83 800 880 1 4.8 35 Yes 598 Comp. Ex. 9 C 80 0.7 20 85 800 880 1 2.5  7 Yes 820 Inv. Ex. 10 C 80 0.8 50 82 800 880 1 1.7  5 Yes 868 Inv. Ex. 11 C 80 0.08 20 7 800 880 1 6.5 35 Yes 598 Comp. Ex. 12 C 80 0.8 0 86 3000 880 1 3.5 20 Yes 545 Comp. Ex. 13 C 80 0.8 10 84 200 900 1 8.9 45 Yes 548 Comp. Ex. 14 C 80 0.8 20 85 800 700 0 Un- Un- Yes 229 Comp. Ex. detectable detectable due to in- due to in- complete complete hardening hardening 15 C 80 0.8 20 86 800 1050 1 15.5  50 Yes 535 Comp. Ex. 16 D 80 0.8 20 79 800 890 1 4.5 15 Yes 830 Inv. Ex. 17 E 80 0.7 20 9 800 890 1 7.5 45 Yes 463 Comp. Ex. 18 F 80 0.7 20 75 800 890 1 4.5 35 Yes 540 Comp. Ex. 19 G 80 0.7 20 70 800 890 1 4.8 32 Yes 651 Comp. Ex. 20 H 80 0.7 20 10 800 890 1 4.2 35 Yes 456 Comp. Ex. 21 A 80 0.8 20 75 2000 800 1 2.8  9 Yes 789 Inv. Ex. 22 I 80 0.8 20 68 800 890 1 4.2 12 Yes 783 Inv. Ex. 23 J 80 0.8 20 82 800 890 1 4.3 12 Yes 785 Inv. Ex. 24 K 80 0.8 20 79 800 890 1 3.9 11 Yes 787 Inv. Ex. 25 L 80 0.8 20 79 800 890 1 4.3 12 Yes 789 Inv. Ex. 26 M 80 0.8 20 79 800 890 1 4.2 12 Yes 791 Inv. Ex. 27 N 80 0.8 20 79 800 890 1 4.3 12 Yes 794 Inv. Ex. *2: If value in the column is zero, the spline is formed by cutting. Otherwise, the spline is formed by rolling and the figure in the column indicates the processing ratio. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

TABLE 7-2 Hot-working Hardened conditions Total Pre- layer structure Total processing hardening HF hardening conditions Average Maximum processing Cooling ratio (%) structure Retention former former Torsional ratio (%) rate at less Proportion Heating time austenitic austenitic fatigue at (° C./s) than (vol. %) Heating temper- (sec) at grain grain strength Steel 800° C. to after 800° C. of bainite rate ature 800° C. diameter diameter Tem- (×10⁵) No. type 1000° C. working *2 structure (° C./s) (° C.) or more (μm) (μm) pering (MPa) Reference 28 O 80 0.8 20 72 800 890 1 3.9 11 Yes 797 Inv. Ex. 29 P 80 0.8 20 79 800 890 1 4.3 12 Yes 800 Inv. Ex. 30 Q 80 0.8 20 79 800 890 1 3.8 10 Yes 803 Inv. Ex. 31 R 80 0.8 20 79 800 890 1 4.3 12 Yes 806 Inv. Ex. 32 S 80 0.8 20 70 800 890 1 4.2 12 Yes 784 Inv. Ex. AJ 0.45 0.45 0.82 0.011 0.031 0.002 0.004 0.4Mo, 26.9 12 Yes 813 Inv. Ex. 0.02Ti, 0.0020B, 0.003Ca AK 0.43 0.51 0.79 0.011 0.06 0.002 0.0039 0.45Mo, 22.6 13 Yes 783 Inv. Ex. 0.021Ti, 0.0020B, 0.003Ca 35 V 80 0.8 20 79 800 890 1 4.3 12 Yes 804 Inv. Ex. 36 W 80 0.8 20 79 800 890 1 3.8 10 Yes 830 Inv. Ex. 37 X 80 0.8 20 79 800 890 1 4.3 12 Yes 794 Inv. Ex. 38 Y 80 0.8 20 79 800 890 1 4.2 12 Yes 797 Inv. Ex. 39 Z 80 0.8 20 79 800 890 1 4.3 12 Yes 800 Inv. Ex. 40 AA 80 0.8 20 85 800 890 1 3.9 11 Yes 803 Inv. Ex. 41 AB 80 0.8 20 79 800 890 1 4.3 12 Yes 806 Inv. Ex. 42 AC 80 0.8 20 80 800 890 1 3.8 10 Yes 784 Inv. Ex. 43 AD 80 0.8 20 79 800 890 1 4.3 12 Yes 813 Inv. Ex. 44 AE 80 0.8 20 79 800 890 1 3.9 11 Yes 794 Inv. Ex. 45 AF 80 0.8 20 79 800 890 1 4.3 12 Yes 797 Inv. Ex. 46 AG 80 0.8 20 79 800 890 1 3.8 10 Yes 800 Inv. Ex. 47 AH 80 0.8 20 79 800 890 1 4.2 12 Yes 803 Inv. Ex. 48 AI 80 0.8 20 85 800 890 1 3.7 10 Yes 806 Inv. Ex. 49 C 80 0.8 50 82 800 880 1 1.7 5 No 950 Inv. Ex. 50 AJ 80 0.8 20 83 800 930 2 3.1 10 Yes 810 Inv. Ex. 51 AK 80 0.8 20 81 500 930 3 3.9 10 Yes 780 Inv. Ex. 52 AL 80 0.8 20 81 500 930 3 4.0 10 Yes 770 Inv. Ex. *2: If value in the column is zero, the spline is formed by cutting. Otherwise, the spline is formed by rolling and the figure in the column indicates the processing ratio. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

Tables 6-1, 6-2, 7-1, and 7-2 clearly show that every constant velocity joint outer race with a hardened structure in which the average former austenitic grain diameter was 12 μm or less and the maximum grain diameter did not exceed four times the average grain diameter exhibited excellent rolling fatigue properties and torsional fatigue strength.

In contrast, the constant velocity joint outer races with an average former austenitic grain diameter exceeding 12 μm and a maximum grain diameter exceeding four times the average grain diameter exhibited poor rolling fatigue and torsional fatigue properties.

Example 4

A constant velocity joint 12 shown in FIG. 14 for transmitting power from a drive shaft 10 to a hub 11 of a wheel was prepared as the component for a machine structural use according to the present invention.

The constant velocity joint 12 was a combination of an outer race 13 and an inner race 14. In particular, the inner race 14 was movably fixed to the inner side of a mouth 13 a of the outer race 13 via balls 15 fit in a ball tracking groove in the inner surface of the mouth 13 a and was connected to the drive shaft 10; meanwhile, a stem 13 b of the outer race 13 was for example splined to the hub 11 to transmit power from the drive shaft 10 to the hub 11 of the wheel.

Each of steel materials having compositions shown in Table 3 was melted in a converter and cast into a cast slab by continuous casting. The size of the cast slab was 300×400 [mm]. The cast slab was subjected to a breakdown step and rolled into a 150 mm square billet. The billet was then rolled to prepare a steel bar having a diameter of 55 mm.

Next, the steel bar was cut to a predetermined length and formed into a constant velocity joint inner race (outer diameter: 45 mm, inner diameter: 20 mm) by hot-forging. The fitting surface thereof was cut or rolled to form riffles for splining. A surface for contact rolling with balls was formed by cutting or cold forging. The cooling conditions after the hot-forging are shown in Tables 8-1, 8-2, 9-1, and 9-2. The total processing ratio in the hot-forging and cold forging was adjusted by controlling the reduction ratio of area of a cross-section taken in the direction orthogonal to the axis direction of the contact rolling surface.

As shown in FIG. 15 or 16, a fitting surface 14 b of the constant velocity joint inner race into which the drive shaft fits and a contact rolling surface 14 a onto which the balls interposed between the inner race and the constant velocity joint outer race make rolling contact were hardened with a 15 Hz high-frequency hardening apparatus under the conditions set forth in Tables 8-1, 8-2, 9-1, and 9-2 to thereby form a hardened structure layer 16. Subsequently, tempering at 180° C. was conducted for 2 hours in a heating furnace to harden the layer. The tempering was omitted for some of the constant velocity joints. The drive shaft was fitted to the fitting surface of the resulting constant velocity joint inner race, and the constant velocity joint inner race was attached to the mouth of the constant velocity joint outer race via balls (steel balls). The stem of the constant velocity joint outer race was fit to the hub to prepare a constant velocity joint unit (see FIG. 11). The specifications for the balls, the outer race, the drive shaft, and the hub were as follows:

balls: hardened and tempered high-carbon chromium bearing steel SUJ2

outer race: high-frequency hardened, tempered carburized steel for machine structural use

hub: high-frequency hardened, tempered carburized steel for machine structural use

drive shaft: high-frequency hardened, tempered carburized steel for machine structural use

Next, using this constant velocity joint, the endurance test for determining rolling fatigue strength was conducted for those in which the fitting surface with the drive shaft was subjected to high-frequency hardening, and the endurance test for determining torsional fatigue strength was conducted for those in which the contact rolling surface with the balls was subjected to high-frequency hardening, both the tests being conducted in a power transmission system for transmitting the rotary action of the drive shaft to the hub through the inner race of the constant velocity joint.

In the rolling fatigue test, power transmission was conducted at a torque of 900 N·m, a working angle (the angle defined by the axis line of the inner race and the axis line of the drive shaft) of 20°, and a number of revolution of 300 rpm. The time taken until separation occurred in the contact rolling surface of the constant velocity joint inner race was evaluated as the rolling fatigue strength. The dimensions and shape of the drive shaft, the constant velocity joint outer race, and the like were adjusted so that the constant velocity joint inner race was the weakest portion in the endurance test.

The constant velocity joint inner races prepared under the same conditions were analyzed to determine the average diameter and the maximum diameter of the former austenitic grains in the hardened layers by the above-described process.

The results are shown in Tables 8-1, 8-2, 9-1, and 9-2. TABLE 8-1 Hot-working Hardened conditions Total Pre- layer structure Total processing hardening HF hardening conditions Average Maximum processing Cooling ratio (%) structure Retention former former Sliding ratio (%) rate at less Proportion Heating time austenitic austenitic rolling at (C. °/s) than (vol. %) Heating temper- (sec) at grain grain contact Steel 800° C. to after 800° C. of bainite rate ature 800° C. diameter diameter Tem- fatigue No. type 1000° C. working *3 structure (° C./s) (° C.) or more (μm) (μm) pering life (hr) Reference 1 A 30 0.7 20 73 800 880 1 6.5 35 Yes 79 Comp. Ex. 2 A 80 0.8 20 71 800 890 1 4.8   9.5 Yes 136 Inv. Ex. 3 A 80 0.7 50 72 800 870 1 2.5   5.3 Yes 147 Inv. Ex. 4 A 80 0.06 20 4 800 910 1 13.5  40 Yes 79 Comp. Ex. 5 A 80 0.8 0 72 800 890 1 7.5 48 Yes 80 Comp. Ex. 6 B 80 0.8 20 75 800 870 1 4.8 11 Yes 147 Inv. Ex. 7 C 30 0.7 20 84 800 880 1 5.6 30 Yes 97 Comp. Ex. 8 C 80 0.8 0 83 800 880 1 4.8 35 Yes 95 Comp. Ex. 9 C 80 0.7 20 85 800 880 1 2.5  7 Yes 146 Inv. Ex. 10 C 80 0.8 50 82 800 880 1 1.7  5 Yes 145 Inv. Ex. 11 C 80 0.08 20 7 800 880 1 6.5 35 Yes 95 Comp. Ex. 12 C 80 0.8 0 86 3000 880 1 3.5 20 Yes 89 Comp. Ex. 13 C 80 0.8 10 84 200 900 1 8.9 45 Yes 88 Comp. Ex. 14 C 80 0.8 20 85 800 700 0 Undetectable Undetect- Yes 49 Comp. Ex. due to able due incomplete to in- hardening complete hardening 15 C 80 0.8 20 86 800 1050 1 15.5  50 Yes 86 Comp. Ex. 16 D 80 0.8 20 79 800 890 1 4.5 15 Yes 148 Inv. Ex. 17 E 80 0.7 20 9 800 890 1 7.5 45 Yes 76 Comp. Ex. 18 F 80 0.7 20 75 800 890 1 4.5 35 Yes 87 Comp. Ex. 19 G 80 0.7 20 70 800 890 1 4.8 32 Yes 93 Comp. Ex. 20 H 80 0.7 20 10 800 890 1 4.2 35 Yes 73 Comp. Ex. 21 A 80 0.8 20 75 2000 800 1 2.8  9 Yes 135 Inv. Ex. 22 I 80 0.8 20 68 800 890 1 4.2 12 Yes 136 Inv. Ex. 23 J 80 0.8 20 82 800 890 1 4.3 12 Yes 135 Inv. Ex. 24 K 80 0.8 20 79 800 890 1 3.9 11 Yes 137 Inv. Ex. 25 L 80 0.8 20 79 800 890 1 4.3 12 Yes 137 Inv. Ex. 26 M 80 0.8 20 79 800 890 1 4.2 12 Yes 139 Inv. Ex. 27 N 80 0.8 20 79 800 890 1 4.3 12 Yes 139 Inv. Ex. *3: If value in the column is zero, the spline is formed by cutting. Otherwise, the spline is formed by rolling and the figure in the column indicates the processing ratio. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

TABLE 8-2 Hot-working Hardened conditions Total Pre- layer structure Total processing hardening HF hardening conditions Average Maximum processing Cooling ratio (%) structure Retention former former Sliding ratio (%) rate at less Proportion Heating time austenitic austenitic rolling at (° C./s) than (vol. %) Heating temper- (sec) at grain grain contact Steel 800° C. to after 800° C. of bainite rate ature 800° C. diameter diameter Tem- fatigue No. type 1000° C. working *3 structure (° C./s) (° C.) or more (μm) (μm) pering life (hr) Reference 28 O 80 0.8 20 72 800 890 1 3.9 11 Yes 139 Inv. Ex. 29 P 80 0.8 20 79 800 890 1 4.3 12 Yes 140 Inv. Ex. 30 Q 80 0.8 20 79 800 890 1 3.8 10 Yes 141 Inv. Ex. 31 R 80 0.8 20 79 800 890 1 4.3 12 Yes 142 Inv. Ex. 32 S 80 0.8 20 70 800 890 1 4.2 12 Yes 142 Inv. Ex. AJ 0.45 0.45 0.82 0.011 0.031 0.002 0.004 0.4Mo, 26.9 12 Yes 143 Inv. Ex. 0.02Ti, 0.0020B, 0.003Ca AK 0.43 0.51 0.79 0.011 0.06 0.002 0.0039 0.45Mo, 22.6 13 Yes 136 Inv. Ex. 0.021Ti, 0.0020B, 0.003Ca 35 V 80 0.8 20 79 800 890 1 4.3 12 Yes 139 Inv. Ex. 36 W 80 0.8 20 79 800 890 1 3.8 10 Yes 149 Inv. Ex. 37 X 80 0.8 20 79 800 890 1 4.3 12 Yes 139 Inv. Ex. 38 Y 80 0.8 20 79 800 890 1 4.2 12 Yes 140 Inv. Ex. 39 Z 80 0.8 20 79 800 890 1 4.3 12 Yes 140 Inv. Ex. 40 AA 80 0.8 20 85 800 890 1 3.9 11 Yes 141 Inv. Ex. 41 AB 80 0.8 20 79 800 890 1 4.3 12 Yes 143 Inv. Ex. 42 AC 80 0.8 20 80 800 890 1 3.8 10 Yes 136 Inv. Ex. 43 AD 80 0.8 20 79 800 890 1 4.3 12 Yes 143 Inv. Ex. 44 AE 80 0.8 20 79 800 890 1 3.9 11 Yes 140 Inv. Ex. 45 AF 80 0.8 20 79 800 890 1 4.3 12 Yes 139 Inv. Ex. 46 AG 80 0.8 20 79 800 890 1 3.8 10 Yes 140 Inv. Ex. 47 AH 80 0.8 20 79 800 890 1 4.2 12 Yes 141 Inv. Ex. 48 AI 80 0.8 20 85 800 890 1 3.7 10 Yes 140 Inv. Ex. 49 C 80 0.8 50 82 800 880 1 1.7 5 No 160 Inv. Ex. 50 AJ 80 0.8 20 83 800 930 2 3.1 10 Yes 145 Inv. Ex. 51 AK 80 0.8 20 81 500 930 3 3.9 10 Yes 138 Inv. Ex. 52 AL 80 0.8 20 81 500 930 3 4.0 10 Yes 135 Inv. Ex. *3: If value in the column is zero, the spline is formed by cutting. Otherwise, the spline is formed by rolling and the figure in the column indicates the processing ratio. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

TABLE 9-1 Hot-working Hardened conditions Total Pre- layer structure Total processing hardening HF hardening conditions Average Maximum processing Cooling ratio (%) structure Retention former former ratio (%) rate at less Proportion Heating time austenitic austenitic at (° C./s) than (vol. %) Heating temper- (sec) at grain grain Rolling Steel 800° C. to after 800° C. of bainite rate ature 800° C. diameter diameter Tem- fatigue No. type 1000° C. working *4 structure (° C./s) (° C.) or more (μm) (μm) pering life (hr) Reference 1 A 30 0.7 20 73 800 880 1 6.5 35 Yes 79 Comp. Ex. 2 A 80 0.8 20 71 800 890 1 4.8   9.5 Yes 136 Inv. Ex. 3 A 80 0.7 50 72 800 870 1 2.5   5.3 Yes 147 Inv. Ex. 4 A 80 0.06 20 4 800 910 1 13.5  40 Yes 79 Comp. Ex. 5 A 80 0.8 0 72 800 890 1 7.5 48 Yes 81 Comp. Ex. 6 B 80 0.8 20 75 800 870 1 4.8 11 Yes 145 Inv. Ex. 7 C 30 0.7 20 84 800 880 1 5.6 30 Yes 99 Comp. Ex. 8 C 80 0.8 0 83 800 880 1 4.8 35 Yes 97 Comp. Ex. 9 C 80 0.7 20 85 800 880 1 2.5  7 Yes 145 Inv. Ex. 10 C 80 0.8 50 82 800 880  1 1.7  5 Yes 155 Inv. Ex. 11 C 80 0.08 20 7 800 880 1 6.5 35 Yes 96 Comp. Ex. 12 C 80 0.8 0 86 3000 880 1 3.5 20 Yes 89 Comp. Ex. 13 C 80 0.8 10 84 200 900 1 8.9 45 Yes 88 Comp. Ex. 14 C 80 0.8 20 85 800 700 0 Undetectable Undetect- Yes 48 Comp. Ex. due to able due incomplete to in- hardening complete hardening 15 C 80 0.8 20 86 800 1050 1 15.5  50 Yes 87 Comp. Ex. 16 D 80 0.8 20 79 800 890 1 4.5 15 Yes 147 Inv. Ex. 17 E 80 0.7 20 9 800 890 1 7.5 45 Yes 76 Comp. Ex. 18 F 80 0.7 20 75 800 890 1 4.5 35 Yes 88 Comp. Ex. 19 G 80 0.7 20 70 800 890 1 4.8 32 Yes 97 Comp. Ex. 20 H 80 0.7 20 10 800 890 1 4.2 35 Yes 75 Comp. Ex. 21 A 80 0.8 20 75 2000 800 1 2.8  9 Yes 136 Inv. Ex. 22 I 80 0.8 20 68 800 890 1 4.2 12 Yes 136 Inv. Ex. 23 J 80 0.8 20 82 800 890 1 4.3 12 Yes 136 Inv. Ex. 24 K 80 0.8 20 79 800 890 1 3.9 11 Yes 137 Inv. Ex. 25 L 80 0.8 20 79 800 890 1 4.3 12 Yes 137 Inv. Ex. 26 M 80 0.8 20 79 800 890 1 4.2 12 Yes 138 Inv. Ex. 27 N 80 0.8 20 79 800 890 1 4.3 12 Yes 139 Inv. Ex. 28 O 80 0.8 20 72 800 890 1 3.9 11 Yes 139 Inv. Ex. 29 P 80 0.8 20 79 800 890 1 4.3 12 Yes 139 Inv. Ex. *4: If the value in the column is zero, the rolling-contact surface for balls is formed by cutting. Otherwise, the rolling-contact surface for balls is formed by cold-forging and the figure in the column indicates the processing ratio. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

TABLE 10-2 Hot-working Working Hardened conditions (cold- Pre- layer structure Total working) hardening HF hardening conditions Average Maximum processing Cooling ratio (%) structure Retention former former ratio (%) rate at less Proportion Heating time austenitic austenitic at (° C./s) than (vol. %) Heating temper- (sec) at grain grain Rolling Steel 800° C. to after 800° C. of bainite rate ature 800° C. diameter diameter Tem- fatigue No. type 1000° C. working *5 structure (° C./s) (° C.) or more (μm) (μm) pering life Reference 29 O 80 0.8 20 72 800 890 1 3.9 11 Yes 10.7 Inv. Ex. 30 P 80 0.8 20 79 800 890 1 4.3 12 Yes 10.9 Inv. Ex. 31 Q 80 0.8 20 79 800 890 1 3.8 10 Yes 11.1 Inv. Ex. 32 R 80 0.8 20 79 800 890 1 4.3 12 Yes 11.3 Inv. Ex. AJ 0.45 0.45 0.82 0.011 0.031 0.002 0.004 0.4Mo, 26.9 12 Yes 11.3 Inv. Ex. 0.02Ti, 0.0020B, 0.003Ca AK 0.43 0.51 0.79 0.011 0.06 0.002 0.0039 0.45Mo, 22.6 12 Yes 11.5 Inv. Ex. 0.021Ti, 0.0020B, 0.003Ca 35 U 80 0.8 20 79 800 890 1 4.5 13 Yes 10.1 Inv. Ex. 36 V 80 0.8 20 79 800 890 1 4.3 12 Yes 10.7 Inv. Ex. 37 W 80 0.8 20 79 800 890 1 3.8 10 Yes 12.7 Inv. Ex. 38 X 80 0.8 20 79 800 890 1 4.3 12 Yes 10.6 Inv. Ex. 39 Y 80 0.8 20 79 800 890 1 4.2 12 Yes 10.9 Inv. Ex. 40 Z 80 0.8 20 79 800 890 1 4.3 12 Yes 10.9 Inv. Ex. 41 AA 80 0.8 20 85 800 890 1 3.9 11 Yes 11.1 Inv. Ex. 42 AB 80 0.8 20 79 800 890 1 4.3 12 Yes 11.5 Inv. Ex. 43 AC 80 0.8 20 80 800 890 1 3.8 10 Yes 10.1 Inv. Ex. 44 AD 80 0.8 20 79 800 890 1 4.3 12 Yes 11.5 Inv. Ex. 45 AE 80 0.8 20 79 800 890 1 3.9 11 Yes 10.9 Inv. Ex. 46 AF 80 0.8 20 79 800 890 1 4.3 12 Yes 10.7 Inv. Ex. 47 AG 80 0.8 20 79 800 890 1 3.8 10 Yes 10.9 Inv. Ex. 48 AH 80 0.8 20 79 800 890 1 4.2 12 Yes 11.1 Inv. Ex. 49 AI 80 0.8 20 85 800 890 1 3.7 10 Yes 10.9 Inv. Ex. 49 C 80 0.8 50 82 800 880 1 1.7 5 No 13.5 Inv. Ex. 50 AJ 80 0.8 20 83 800 930 2 3.1 10 Yes 11.1 Inv. Ex. 51 AK 80 0.8 20 81 500 930 3 3.9 10 Yes 10.4 Inv. Ex. 52 AL 80 0.8 20 81 500 930 3 4.0 10 Yes 10.3 Inv. Ex.. *5: If the value in the column is zero, the rolling-contact surface for balls is formed by cutting. Otherwise, the rolling-contact surface for balls is formed by cold-forging and the figure in the column indicates the processing ratio. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

TABLE 9-2 Hot-working Hardened conditions Total Pre- layer structure Total processing hardening HF hardening conditions Average Maximum processing Cooling ratio (%) structure Retention former former ratio (%) rate at less Proportion Heating time austenitic austenitic at (° C./s) than (vol. %) Heating temper- (sec) at grain grain Rolling Steel 800° C. to after 800° C. of bainite rate ature 800° C. diameter diameter Tem- fatigue No. type 1000° C. working *4 structure (° C./s) (° C.) or more (μm) (μm) pering life (hr) Reference 30 Q 80 0.8 20 79 800 890 1 3.8 10 Yes 141 Inv. Ex. 31 R 80 0.8 20 79 800 890 1 4.3 12 Yes 141 Inv. Ex. 32 S 80 0.8 20 70 800 890 1 4.2 12 Yes 136 Inv. Ex. AJ 0.45 0.45 0.82 0.011 0.031 0.002 0.004 0.4Mo, 26.9 12 Yes 143 Inv. Ex. 0.02Ti, 0.0020B, 0.003Ca AK 0.43 0.51 0.79 0.011 0.06 0.002 0.0039 0.45Mo, 22.6 13 Yes 136 Inv. Ex. 0.021Ti, 0.0020B, 0.003Ca 35 V 80 0.8 20 79 800 890 1 4.3 12 Yes 141 Inv. Ex. 36 W 80 0.8 20 79 800 890 1 3.8 10 Yes 147 Inv. Ex. 37 X 80 0.8 20 79 800 890 1 4.3 12 Yes 139 Inv. Ex. 38 Y 80 0.8 20 79 800 890 1 4.2 12 Yes 139 Inv. Ex. 39 Z 80 0.8 20 79 800 890 1 4.3 12 Yes 140 Inv. Ex. 40 AA 80 0.8 20 85 800 890 1 3.9 11 Yes 141 Inv. Ex. 41 AB 80 0.8 20 79 800 890 1 4.3 12 Yes 142 Inv. Ex. 42 AC 80 0.8 20 80 800 890 1 3.8 10 Yes 136 Inv. Ex. 43 AD 80 0.8 20 79 800 890 1 4.3 12 Yes 143 Inv. Ex. 44 AE 80 0.8 20 79 800 890 1 3.9 11 Yes 138 Inv. Ex. 45 AF 80 0.8 20 79 800 890 1 4.3 12 Yes 139 Inv. Ex. 46 AG 80 0.8 20 79 800 890 1 3.8 10 Yes 139 Inv. Ex. 47 AH 80 0.8 20 79 800 890 1 4.2 12 Yes 141 Inv. Ex. 48 AI 80 0.8 20 85 800 890 1 3.7 10 Yes 141 Inv. Ex. 49 C 80 0.8 50 82 800 880 1 1.7 5 No 170 Inv. Ex. 50 AJ 80 0.8 20 83 800 930 2 3.1 10 Yes 142 Inv. Ex. 51 AK 80 0.8 20 81 500 930 3 3.9 10 Yes 137 Inv. Ex. 52 AL 80 0.8 20 81 500 930 3 4.0 10 Yes 135 Inv. Ex. *4: If the value in the column is zero, the rolling-contact surface for balls is formed by cutting. Otherwise, the rolling-contact surface for balls is formed by cold-forging and the figure in the column indicates the processing ratio. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

Tables 8-1, 8-2, 9-1, and 9-2 clearly show that every constant velocity joint inner race with a hardened structure in which the average former austenitic grain diameter was 12 μm or less and the maximum grain diameter did not exceed four times the average grain diameter exhibited excellent fatigue properties.

In contrast, the constant velocity joint inner races with an average former austenitic grain diameter exceeding 12 μm and a maximum grain diameter exceeding four times the average grain diameter exhibited poor fatigue properties.

Example 5

A hub shown in FIG. 17 for an automobile wheel was prepared as a component for machine structural use according to the present invention.

A hub 17 for an automobile wheel has a shaft 18 also serving as the inner race of a bearing and constitutes the bearing via balls 21 interposed between an outer race 20 and the outer surface of the shaft 18. In FIG. 17, reference numeral 19 denotes a spacer for retaining the balls 21 between the shaft 18 of the hub and the outer race 20. In the illustration of FIG. 17, the outer surface (contact rolling surface) 22 onto which the balls serving as a bearing for the hub make rolling contact requires improved rolling fatigue life.

Each of steel materials having compositions shown in Table 3 was melted in a converter and cast into a cast slab by continuous casting. The size of the cast slab was 300×400 [mm]. The cast slab was subjected to a breakdown step and rolled into a 150 mm square billet. The billet was then rolled to prepare a steel bar having a diameter of 24 mm. The steel bar was then cut to a predetermined length, formed into the shape of a hub by hot-forging, and cooled at a cooling rate set forth in Tables 10-1 and 10-2. The outer surface onto which the bearing balls for the hub shaft make rolling contact was formed by cutting or cold forging and subjected to high-frequency hardening under the conditions set forth in Tables 10-1 and 10-2 to form a hardened structure layer. Then the hub was tempered at 170° C. for 30 minutes in a heating furnace and subjected to finishing to prepare a product. Tempering was omitted for some of the hubs. The total processing ratio in the hot-forging and cold forging was controlled by adjusting the ratio of change in area of the cross-section taken in the direction orthogonal to the axis direction of the contact rolling surface.

The rolling fatigue life of each resulting hub was analyzed and the results are shown in Tables 10-1 and 10-2.

The rolling fatigue life of the hub was evaluated as follows.

Bearing balls were arranged on the outer surface of the shaft of the hub, and an outer race was attached. While fixing the hub, a predetermined load (900 N) was applied onto the outer race 20 of the hub as shown in FIG. 17 and the outer race 20 of the hub was rotated at a predetermined rate (300 rpm) to conduct the endurance test. The time taken until occurrence of the rolling fatigue fracture of a high-frequency hardened structure layer 22 was evaluated as the rolling fatigue life.

The rolling fatigue life was indicated in terms of the ratio with respect to the rolling fatigue life of Sample No. 22, which is a related-art sample prepared under the hot working and high-frequency hardening conditions outside the range of the present invention, in Tables 10-1 and 10-2, the rolling fatigue life of the related-art sample being defined as 1.

Here, the dimensions and shape of the outer race, steel balls, and the like were adjusted so that the shaft contact rolling surface of the hub was the weakest part in the endurance test.

The same hub was analyzed to determine the average diameter and the maximum diameter of the former austenitic grains in the hardened structure layer by the above-described process.

The results are also shown in Tables 10-1 and 10-2. TABLE 10-1 Hot-working Working Hardened conditions (cold- Pre- layer structure Total working) hardening HF hardening conditions Average Maximum processing Cooling ratio (%) structure Retention former former ratio (%) rate at less Proportion Heating time austenitic austenitic at (° C./s) than (vol. %) Heating temper- (sec) at grain grain Rolling Steel 800° C. to after 800° C. of bainite rate ature 800° C. diameter diameter Tem- fatigue No. type 1000° C. working *5 structure (° C./s) (° C.) or more (μm) (μm) pering life Reference 1 A 30 0.7 20 73 800 880 1 6.5 35 Yes 2.5 Comp. Ex. 2 A 80 0.8 20 71 800 890 1 4.8   9.5 Yes 10.1 Inv. Ex. 3 A 80 0.7 50 72 800 870 1 2.5   5.3 Yes 12.3 Inv. Ex. 4 A 80 0.06 20 4 800 910 1 13.5  40 Yes 2.6 Comp. Ex. 5 A 80 0.8 0 72 800 890 1 7.5 48 Yes 2.6 Comp. Ex. 6 B 80 0.8 20 6 800 870 1 4.8 11 Yes 12.4 Inv. Ex. 7 C 30 0.7 20 84 800 880 1 5.6 30 Yes 4.3 Comp. Ex. 8 C 80 0.8 0 83 800 880 1 4.8 35 Yes 4.1 Comp. Ex. 9 C 80 0.7 20 85 800 880 1 2.5  7 Yes 12.1 Inv. Ex. 10 C 80 0.8 50 82 800 880 1 1.7  5 Yes 12.3 Inv. Ex. 11 C 80 0.08 20 7 800 880 1 6.5 35 Yes 4.1 Comp. Ex. 12 C 80 0.8 0 86 3000 880 1 3.5 20 Yes 3.4 Comp. Ex. 13 C 80 0.8 10 84 200 900 1 8.9 45 Yes 3.4 Comp. Ex. 14 C 80 0.8 20 85 800 700 0 Undetectable Undetect- Yes 0.8 Comp. Ex. due to able due incomplete to in- hardening complete hardening 15 C 80 0.8 20 86 800 1050 1 15.5  50 Yes 3.2 Comp. Ex. 16 D 80 0.8 20 79 800 890 1 4.5 15 Yes 12.5 Inv. Ex. 17 E 80 0.7 20 9 800 890 1 7.5 45 Yes 2.3 Comp. Ex. 18 F 80 0.7 20 75 800 890 1 4.5 35 Yes 3.3 Comp. Ex. 19 G 80 0.7 20 70 800 890 1 4.8 32 Yes 3.9 Comp. Ex. 20 H 80 0.7 20 10 800 890 1 4.2 35 Yes 2.1 Comp. Ex. 21 A 80 0.8 20 75 2000 800 1 2.8  9 Yes 4.0 Comp. Ex. 22 A 80 0.1 0 0 200 1050 7 17   88 Yes 1.0 Rel. Ex. 23 I 80 0.8 20 68 800 890 1 4.2 12 Yes 10.1 Inv. Ex. 24 J 80 0.8 20 82 800 890 1 4.3 12 Yes 10.1 Inv. Ex. 25 K 80 0.8 20 79 800 890 1 3.9 11 Yes 10.3 Inv. Ex. 26 L 80 0.8 20 79 800 890 1 4.3 12 Yes 10.4 Inv. Ex. 27 M 80 0.8 20 79 800 890 1 4.2 12 Yes 10.7 Inv. Ex. 28 N 80 0.8 20 79 800 890 1 4.3 12 Yes 10.6 Inv. Ex. *5: If the value in the column is zero, the rolling-contact surface for balls is formed by cutting. Otherwise, the rolling-contact surface for balls is formed by cold-forging and the figure in the column indicates the processing ratio. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

Tables 10-1 and 10-2 clearly show that every hub with a hardened structure in which the average former austenitic grain diameter was 12 μm or less and the maximum grain diameter did not exceed four times the average grain diameter exhibited a rolling fatigue life at least 10 times that of the related art sample.

In contrast, Comparative Examples with an average former austenitic grain diameter exceeding 12 μm and a maximum grain diameter exceeding four times the average grain diameter exhibited short rolling fatigue life.

Example 6

A hub shown in FIG. 18 was prepared as in EXAMPLE 5 as an component for machine structural use according to the present invention. In particular, each of steel materials having compositions shown in Table 3 was melted in a converter and cast into a cast slab by continuous casting. The size of the cast slab was 300×400 [mm]. The cast slab was subjected to a breakdown step, rolled into a 150 mm square billet, and rolled to prepare a steel bar having a diameter of 24 mm. The steel bar was cut to a predetermined length, formed into a shape of a hub by hot-forging, and cooled at a rate shown in Tables 11-1 and 11-2. Subsequently, the hub shaft was cut or rolled for splining for fitting with the shaft of the constant velocity joint.

The inner surface (fitting portion 23 in FIG. 18) of the hub shaft that fits with the shaft of the constant velocity joint was subjected to high-frequency hardening under the conditions set forth in Tables 10-1 and 10-2 to form a hardened structure layer. The hub was then tempered at 170° C. for 30 minutes in a heating furnace and subjected to finishing to prepare a product. Tempering was omitted for some of the hubs. The total processing ratio in the hot-forging and forming rolling was adjusted by controlling the ratio of change in cross-section in the axis direction of the fitting portion of the hub shaft with the constant velocity joint.

The resulting hub was analyzed to determine the sliding rolling contact fatigue life of the inner surface that fits with the constant velocity joint. The results are shown in Tables 11-1 and 11-2.

The sliding rolling contact fatigue life of the hub was evaluated as follows:

Sliding Rolling Contact Fatigue Life

As shown in FIG. 19, a shaft 24 of the constant velocity joint was fit into the inner surface of the shaft of the hub, and while fixing the hub, fully reversed cycles of torsional force were applied to the shaft of the constant velocity joint (maximum torque: 700 N, 2 cycles per second). The number of cycles until occurrence of fracture of the hub spline due to the sliding rolling contact fatigue was evaluated as the fatigue life.

The sliding rolling contact fatigue life was indicated in terms of the ratio with respect to the sliding rolling contact fatigue life of Sample No. 22, which is a related art sample prepared under the hot-working and high-frequency hardening conditions outside the range of the present invention), in Tables 11-1 and 11-2, the sliding rolling contact fatigue life of the related-art sample being defined as 1.

The same hub was analyzed to determine the average diameter and the maximum diameter of the former austenitic grains in the hardened structure layer by the above-described process.

The results are also set forth in Tables 11-1 and 11-2. TABLE 11-1 Hot-working Working Hardened conditions (cold- Pre- layer structure Total working) hardening HF hardening conditions Average Maximum processing Cooling ratio (%) structure Retention former former Sliding ratio (%) rate at less Proportion Heating time austenitic austenitic rolling at (° C./s) than (vol. %) Heating temper- (sec) at grain grain contact Steel 800° C. to after 800° C. of bainite rate ature 800° C. diameter diameter Tem- fatigue No. type 1000° C. working *6 structure (° C./s) (° C.) or more (μm) (μm) pering life Reference 1 A 30 0.7 20 73 800 880 1 6.5 35 Yes 2.5 Comp. Ex. 2 A 80 0.8 20 71 800 890 1 4.8   9.5 Yes 10.3 Inv. Ex. 3 A 80 0.7 50 72 800 870 1 2.5   5.3 Yes 12.3 Inv. Ex. 4 A 80 0.06 20 4 800 910 1 13.5  40 Yes 2.6 Comp. Ex. 5 A 80 0.8 0 72 800 890 1 7.5 48 Yes 2.6 Comp. Ex. 6 B 80 0.8 20 75 800 870 1 4.8 11 Yes 12.6 Inv. Ex. 7 C 30 0.7 20 84 800 880 1 5.6 30 Yes 3.9 Comp. Ex. 8 C 80 0.8 0 83 800 880 1 4.8 35 Yes 3.9 Comp. Ex. 9 C 80 0.7 20 85 800 880 1 2.5  7 Yes 12.1 Inv. Ex. 10 C 80 0.8 50 82 800 880 1 1.7  5 Yes 12.3 Inv. Ex. 11 C 80 0.08 20 7 800 880 1 6.5 35 Yes 3.8 Comp. Ex. 12 C 80 0.8 0 86 3000 880 1 3.5 20 Yes 3.4 Comp. Ex. 13 C 80 0.8 10 84 200 900 1 8.9 45 Yes 3.3 Comp. Ex. 14 C 80 0.8 20 85 800 700 0 Undetectable Undetect- Yes 0.5 Comp. Ex. due to able due incomplete to in- hardening complete hardening 15 C 80 0.8 20 86 800 1050 1 15.5  50 Yes 3.2 Comp. Ex. 16 D 80 0.8 20 79 800 890 1 4.5 15 Yes 12.5 Inv. Ex. 17 E 80 0.7 20 9 800 890 1 7.5 45 Yes 2.3 Comp. Ex. 18 F 80 0.7 20 75 800 890 1 4.5 35 Yes 3.3 Comp. Ex. 19 G 80 0.7 20 70 800 890 1 4.8 32 Yes 3.9 Comp. Ex. 20 H 80 0.7 20 10 800 890 1 4.2 35 Yes 2.1 Comp. Ex. 21 A 80 0.8 20 75 2000 800 1 2.8  9 Yes 4.0 Comp. Ex. 22 A 80 0.1 0 0 200 1050 7 17   88 Yes 1.0 Rel. Ex. 23 I 80 0.8 20 68 800 890 1 4.2 12 Yes 10.1 Inv. Ex. 24 J 80 0.8 20 82 800 890 1 4.3 12 Yes 10.1 Inv. Ex. *6: If the value in the column is zero, the spline is formed by cutting. Otherwise, the spline is formed by rolling and the figure in the column indicates the processing ratio Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

TABLE 11-2 Pre-hardening Hot-working conditions Working (cold- structure HF hardening conditions Total processing Cooling rate working) ratio Proportion Heating Retention time Steel ratio (%) at (° C./s) after (%) at less than (vol. %) of bainite Heating temperature (sec) at 800° C. No. type 800° C. to 1000° C. working 800° C. *6 structure rate(° C./s) (° C.) or more 25 K 80 0.8 20 79 800 890 1 26 L 80 0.8 20 79 800 890 1 27 M 80 0.8 20 79 800 890 1 28 N 80 0.8 20 79 800 890 1 29 O 80 0.8 20 72 800 890 1 30 P 80 0.8 20 79 800 890 1 31 Q 80 0.8 20 79 800 890 1 32 R 80 0.8 20 79 800 890 1 AJ 0.45 0.45 0.82 0.011 0.031 0.002 0.004 0.4Mo, 0.02Ti, 0.0020B, 0.003Ca AK 0.43 0.51 0.79 0.011 0.06 0.002 0.0039 0.45Mo, 0.021Ti, 0.0020B, 0.003Ca 35 U 80 0.8 20 79 800 890 1 36 V 80 0.8 20 79 800 890 1 37 W 80 0.8 20 79 800 890 1 38 X 80 0.8 20 79 800 890 1 39 Y 80 0.8 20 79 800 890 1 40 Z 80 0.8 20 79 800 890 1 41 AA 80 0.8 20 85 800 890 1 42 AB 80 0.8 20 79 800 890 1 43 AC 80 0.8 20 80 800 890 1 44 AD 80 0.8 20 79 800 890 1 45 AE 80 0.8 20 79 800 890 1 46 AF 80 0.8 20 79 800 890 1 47 AG 80 0.8 20 79 800 890 1 48 AH 80 0.8 20 79 800 890 1 49 AI 80 0.8 20 85 800 890 1 50 C 80 0.8 50 82 800 880 1 51 AJ 80 0.8 20 83 800 930 2 52 AK 80 0.8 20 81 500 930 3 53 AL 80 0.8 20 81 500 930 3 Sliding Hardened layer structure rolling Average former Maximum former contact austenitic grain austenitic grain fatigue No. diameter (μm) diameter (μm) Tempering life Reference 25 3.9 11 Yes 10.3 Inv. Ex. 26 4.3 12 Yes 10.4 Inv. Ex. 27 4.2 12 Yes 10.7 Inv. Ex. 28 4.3 12 Yes 10.6 Inv. Ex. 29 3.9 11 Yes 10.3 Inv. Ex. 30 4.3 12 Yes 10.4 Inv. Ex. 31 3.8 10 Yes 10.7 Inv. Ex. 32 4.3 12 Yes 10.6 Inv. Ex. AJ 26.9 12 Yes 11.3 Inv. Ex. AK 22.6 12 Yes 11.5 Inv. Ex. 35 4.5 13 Yes 10.1 Inv. Ex. 36 4.3 12 Yes 10.7 Inv. Ex. 37 3.8 10 Yes 12.7 Inv. Ex. 38 4.3 12 Yes 10.7 Inv. Ex. 39 4.2 12 Yes 10.6 Inv. Ex. 40 4.3 12 Yes 11.3 Inv. Ex. 41 3.9 11 Yes 11.5 Inv. Ex. 42 4.3 12 Yes 10.1 Inv. Ex. 43 3.8 10 Yes 10.1 Inv. Ex. 44 4.3 12 Yes 11.5 Inv. Ex. 45 3.9 11 Yes 10.9 Inv. Ex. 46 4.3 12 Yes 10.7 Inv. Ex. 47 3.8 10 Yes 10.0 Inv. Ex. 48 4.2 12 Yes 11.1 Inv. Ex. 49 3.7 10 Yes 10.8 Inv. Ex. 50 1.7 5 No 13.5 Inv. Ex. 51 3.1 10 Yes 11.1 Inv. Ex. 52 3.9 10 Yes 11.5 Inv. Ex. 53 4.0 10 Yes 11.5 Inv. Ex. *6: If the value in the column is zero, the spline is formed by cutting. Otherwise, the spline is formed by rolling and the figure in the column indicates the processing ratio Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

Tables 11-1 and 11-2 clearly show that every hub with a hardened structure in which the average former austenitic grain diameter was 12 μm or less and the maximum grain diameter did not exceed four times the average grain diameter exhibited a sliding rolling contact fatigue life at least 10 times that of the related-art sample.

In contrast, Comparative Examples with an average former austenitic grain diameter exceeding 12 μm and a maximum grain diameter exceeding four times the average grain diameter exhibited short sliding rolling contact fatigue life.

Example 7

A gear 25 shown in FIG. 20 was prepared as a component for machine structural use according to the present invention.

A gear 25 shown in FIG. 20 was a representative example of a gear and has many teeth 26 in the edge face. The gear of the present invention, as shown in FIG. 21, had many teeth 26 and bottoms 27 between the teeth 26, and a hardened structure layer 28 was formed in the surface layer of the teeth and the bottoms. In the illustrated example, the hardened structure layer 28 was formed in the surface portion of the teeth 26 and the bottoms 27; alternatively, it was possible to form a hardened structure layer in the inner surface of a shaft hole 29 into which a driving shaft of a variety of type was inserted.

Each of steel materials having compositions shown in Table 3 was melted in a converter and cast into a cast slab by continuous casting. The size of the cast slab was 300×400 [mm]. The cast slab was subjected to a breakdown step and rolled into a 150 mm square billet. The billet was then rolled under hot working conditions indicated in Tables 12-1 and 12-2 to prepare a steel bar having a diameter of 90 mm. The processing ratio here is defined as the ratio of reduction in the cross section in the individual temperature ranges.

A gear was prepared from this steel bar by cutting as follows:

small gear: outer diameter: 75 mm, module: 2.5, number of teeth: 28, reference diameter: 70 mm

large gear: outer diameter: 85 mm, module: 2.5, number of teeth: 32, reference diameter: 80 mm

This gear was hardened under the conditions set forth in Tables 12-1 and 12-2 using a 200 kHz high-frequency hardening apparatus and tempered at 180° C. for 2 hours in a heating furnace, followed by fatigue test of actual gears. The tempering was omitted for some of the gears.

The fatigue test of actual gears was conducted by meshing the small and large gears and rotating them at a speed of 3000 rpm and a load torque of 245 N·m. The number of torque application cycles until fracture of one of the gears was evaluated.

The results are set forth in Tables 12-1 and 12-2.

Gears prepared under same conditions were analyzed to determine the average diameter and the maximum diameter of the former austenitic grains in the hardened structure layer by the above-described process.

The results are also set forth in Tables 12-1 and 12-2. TABLE 12-1 Hot-working conditions Pre-hardening Total processing structure HF hardening conditions ratio (%) at Processing ratio Cooling rate Proportion Heating Retention time Steel 800° C. to (%) at 700° C. to (° C./s) after (vol. %) of bainite Heating temperature (sec) at 800° C. No. type 1000° C. less than 800° C. working structure rate(° C./s) (° C.) or more  1 A 30 20 0.7 73 800 880 1  2 A 80 20 0.8 71 800 890 1  3 A 80 50 0.7 72 800 870 1  4 A 80 20 0.06 4 800 910 1  5 A 80 0 0.8 72 800 890 1  6 B 80 20 0.8 75 800 870 1  7 C 30 20 0.7 84 800 880 1  8 C 80 0 0.8 83 800 880 1  9 C 80 20 0.7 85 800 880 1 10 C 80 50 0.8 82 800 880 1 11 C 80 20 0.08 7 800 880 1 12 C 80 0 0.8 86 3000 880 1 13 C 80 10 0.8 84 200 900 1 14 C 80 20 0.8 85 800 700 0 15 C 80 20 0.8 86 800 1050 1 16 D 80 20 0.8 79 800 890 1 17 E 80 20 0.7 9 800 890 1 18 F 80 20 0.7 75 800 890 1 19 G 80 20 0.7 70 800 890 1 20 H 80 20 0.7 10 800 890 1 21 A 80 20 0.8 75 2000 800 1 22 I 80 20 0.8 68 800 890 1 23 J 80 20 0.8 82 800 890 1 Hardened layer structure Number of Average former Maximum former torque austenitic grain austenitic grain application No. diameter (μm) diameter (μm) Tempering (×10⁴) Reference  1 6.5 35 Yes 36 Comp. Ex.  2 4.8   9.5 Yes 1039 Inv. Ex.  3 2.5   5.3 Yes 1691 Inv. Ex.  4 13.5  40 Yes 36 Comp. Ex.  5 7.5 48 Yes 39 Comp. Ex.  6 4.8 11 Yes 1725 Inv. Ex.  7 5.6 30 Yes 128 Comp. Ex.  8 4.8 35 Yes 113 Comp. Ex.  9 2.5  7 Yes 1621 Inv. Ex. 10 1.7  5 Yes 1553 Inv. Ex. 11 6.5 35 Yes 113 Comp. Ex. 12 3.5 20 Yes 73 Comp. Ex. 13 8.9 45 Yes 70 Comp. Ex. 14 Undetectable Undetectable Yes 2 Comp. Ex. due to due  to incomplete incomplete hardening hardening 15 15.5  50 Yes 61 Comp. Ex. 16 4.5 15 Yes 1745 Inv. Ex. 17 7.5 45 Yes 29 Comp. Ex. 18 4.5 35 Yes 65 Comp. Ex. 19 4.8 32 Yes 99 Comp. Ex. 20 4.2 35 Yes 22 Comp. Ex. 21 2.8  9 Yes 61 Comp. Ex. 22 4.2 12 Yes 1745 Inv. Ex. 23 4.3 12 Yes 997 Inv. Ex. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

TABLE 12-2 Hot-working conditions Pre-hardening Total processing structure HF hardening conditions ratio (%) at Processing ratio Cooling rate Proportion Heating Retention time Steel 800° C. to (%) at 700° C. to (° C./s) after (vol. %) of bainite Heating temperature (sec) at 800° C. No. type 1000° C. less than 800° C. working structure rate(° C./s) (° C.) or more 24 K 80 20 0.8 79 800 890 1 25 L 80 20 0.8 79 800 890 1 26 M 80 20 0.8 79 800 890 1 27 N 80 20 0.8 79 800 890 1 28 O 80 20 0.8 72 800 890 1 29 P 80 20 0.8 79 800 890 1 30 Q 80 20 0.8 79 800 890 1 31 R 80 20 0.8 79 800 890 1 32 S 80 20 0.8 70 800 890 1 33 T 80 20 0.8 79 800 890 1 34 U 80 20 0.8 79 800 890 1 35 V 80 20 0.8 79 800 890 1 36 W 80 20 0.8 79 800 890 1 37 X 80 20 0.8 79 800 890 1 38 Y 80 20 0.8 79 800 890 1 39 Z 80 20 0.8 79 800 890 1 40 AA 80 20 0.8 85 800 890 1 41 AB 80 20 0.8 79 800 890 1 42 AC 80 20 0.8 80 800 890 1 43 AD 80 20 0.8 79 800 890 1 44 AE 80 20 0.8 79 800 890 1 45 AF 80 20 0.8 79 800 890 1 46 AG 80 20 0.8 79 800 890 1 47 AH 80 20 0.8 79 800 890 1 48 AI 80 20 0.8 85 800 890 1 49 C 80 50 0.8 82 800 880 1 50 AJ 80 20 0.8 83 800 930 2 51 AK 80 20 0.8 81 500 930 3 52 AL 80 20 0.8 81 500 930 3 Hardened layer structure Number of Average former Maximum former torque austenitic grain austenitic grain application No. diameter (μm) diameter (μm) Tempering (×10⁴) Reference 24 3.9 11 Yes 1089 Inv. Ex. 25 4.3 12 Yes 1115 Inv. Ex. 26 4.2 12 Yes 1195 Inv. Ex. 27 4.3 12 Yes 1169 Inv. Ex. 28 3.9 11 Yes 1211 Inv. Ex. 29 4.3 12 Yes 1254 Inv. Ex. 30 3.8 10 Yes 1300 Inv. Ex. 31 4.3 12 Yes 1364 Inv. Ex. 32 4.2 12 Yes 1364 Inv. Ex. 33 4.3 12 Yes 1445 Inv. Ex. 34 4.5 13 Yes 1039 Inv. Ex. 35 4.3 12 Yes 1195 Inv. Ex. 36 3.8 10 Yes 1839 Inv. Ex. 37 4.3 12 Yes 1169 Inv. Ex. 38 4.2 12 Yes 1249 Inv. Ex. 39 4.3 12 Yes 1254 Inv. Ex. 40 3.9 11 Yes 1300 Inv. Ex. 41 4.3 12 Yes 1425 Inv. Ex. 42 3.8 10 Yes 1051 Inv. Ex. 43 4.3 12 Yes 1445 Inv. Ex. 44 3.9 11 Yes 1249 Inv. Ex. 45 4.3 12 Yes 1211 Inv. Ex. 46 3.8 10 Yes 1254 Inv. Ex. 47 4.2 12 Yes 1300 Inv. Ex. 48 3.7 10 Yes 1249 Inv. Ex. 49 1.7 5 No 1700 Inv. Ex. 50 3.1 10 Yes 1310 Inv. Ex. 51 3.9 10 Yes 1350 Inv. Ex. 52 4.0 10 Yes 1340 Inv. Ex. Rel. Ex.: Example of related art, Inv. Ex.: Example of the present invention Comp. Ex.: Comparative example

Tables 12-1 and 12-2 clearly show that when the hardened layer had an average former austenitic grain diameter of 12 μm or less and a maximum grain diameter not exceeding four times the average grain diameter, excellent fatigue properties, i.e., the number of torque load application cycles being about 1000×10⁴ or more, was achieved.

In contrast, gears having hardened structures with an average former austenitic grain diameter exceeding 12 μm and a maximum grain diameter exceeding four times the average grain diameter exhibited poor fatigue properties.

INDUSTRIAL APPLICABILITY

According to the present invention, a component for machine structural use that excels in all the fatigue properties, such as torsional fatigue property, the bending fatigue property, the rolling fatigue property, and the sliding rolling contact fatigue property, can be stably produced. Therefore, the invention has a significant effect on the demand of weight-reduction of automobile components and the like. 

1. A component for machine structural use, comprising a steel material at least part of which is subjected to hardening, wherein the hardened structure has an average diameter of former austenitic grains of 12 mm or less and a maximum grain diameter not exceeding four times the average grain diameter.
 2. The component for machine structural use according to claim 1, the steel material comprising a composition containing, in terms of percent by mass: C: 0.3% to 1.5%. Si: 0.05% to 3.0% and Mn: 0.2% to 2.0%, and satisfying a formula (1): C^(1/2)(1+0.7Si)(1+3Mn)>2.0   (1) wherein the balance is Fe and inevitable impurities.
 3. The component for machine structural use according to claim 2, the composition further containing, in terms of percent by mass: Al: 0.25% or less.
 4. A component for machine structural use comprising a steel material at least tart of which is subjected to hardening, wherein the hardened structure has an average diameter of former austenitic grains of 12 μm or less and a maximum grain diameter not exceeding four times the average grain diameter, the steel material comprising, a composition containing in terms of percent by mass: C: 0.3% to 1.5%; Si: 0.05% to 3.0%; Mn: 0.2% to 2.0%; at least one element selected from the group consisting of Cr: 0.03% to 2.5%, Mo: 0.05% to 1.0%, Cu: 0.03% to 1.0%, Ni: 0.05% to 2.5%, Co: 0.01% to 1.0%, V: 0.01% to 0.5% and W: 0.005% to 1.0%, and satisfying a formula (2): C^(1/2)(1+0.7Si)(1+3Mn)(1+2.1Cr)(1+3.0 Mo)(1+0.4 Cu)(1+0.3Ni)(1+5.0V)(1+0.5W)>2.0   (2) wherein the balance is Fe and inevitable impurities.
 5. A component for machine structural use comprising a steel material at least part of which is subjected to hardening, wherein the hardened structure has an average diameter of former austenitic grains of 12 μm or less and a maximum grain diameter not exceeding four times the average grain diameter, the steel material comprising, a composition containing, in terms of percent by mass: C: 0.3% to 1.5%; Si: 0.05% to 3.0%; Mn: 0.2% to 2.0%; at least one element selected from the group consisting of Cr: 0,03% to 2.5%, Mo: 0.05% to 1.0%, Cu: 0.03 to 1.0%, Ni: 0.05% to 2.5%, Co: 0.01% to 1.0%, V: 0.01% to 0.5% and W: 0.005% to 1.0%; at least one element selected from the group consisting of Ti: 0.005% to 0.1%, Nb: 0.005% to 0.1%, Zr: 0.005% to 0.1%, B: 0.0003% to 0.01%, Ta: 0.02% to 0.5%, Hf: 0.02% to 0.5% and Sb: 0.005% to 0.015%, and satisfying a formula (3): C^(1/2)(1+0.7Si)(1+3Mn)(1+2.1Cr)(1+3.0 Mo)(1+0.4 Cu)(1+0.3Ni)(1+5.0V)(1+1000B)(1+0.5W)>2.0   (3) wherein the balance is Fe and inevitable impurities.
 6. The component for machine structural use according to claim 2 or 3, the composition further containing, in terms of percent by mass, at least one element selected from the group consisting of S: 0.1% or less, Pb: 0.01% to 0.1%, Bi: 0.01% to 0.1%, Se: 0.003% to 0.1%, Te: 0.003% to 0.1%, Ca: 0.0001% to 0.01%, Mg: 0.0001% to 0.01% and REM: 0.0001% to 0.1%.
 7. A method for making a component for machine structural use, comprising subjecting at least part of a steel material to high-frequency heating at least once, the steel material containing one or both of a fine bainite structure and a fine martensite structure in a total of 10 percent by volume, wherein the high-frequency heating is conducted at a heating rate of at least 400° C./s and an ultimate temperature of 1,000° C. or less.
 8. The method for making the component for machine structural use according to claim 7, wherein the steel material is worked through: a hot-working step in which a total processing ratio at 800° C. to 1,000° C. is 80% or more; a cooling step subsequent to the hot-working step, in which the cooling rate is 0.2° C./s or more in the temperature range of 700° C. to 500° C.; and a secondary working step of either effecting 20% or more of working in a temperature range of 700° C. to less than 800° C. before the cooling step or effecting 20% or more of working in a temperature range of up to the A₁ transformation temperature subsequent to the cooling step.
 9. The method for making the component for machine structural use according to claim 7 or 8, wherein a retention time at 800° C. or more in the high-frequency heating is 5 seconds or less at a time.
 10. The method for making the component for machine structural use according to claim 7 or 8, wherein the steel material comprising, in terms of percent by mass, C: 0.3% to 1.5%; Si: 0.05% to 3.0% and Mn: 0.2% to 2.0%, and satisfying a formula (1): C^(1/2)(1+0.7Si)(1+3Mn)>2.0   (1) wherein the balance is Fe and inevitable impurities.
 11. The method for making the component for machine structural use according to claim 10, the steel material further comprising: Al: 0.25 mass % or less.
 12. The method for making the component for machine structural use according to claim 10, wherein the steel material comprising, in terms of percent by mass, C: 0.3% to 1.5%; Si: O.005% to 3.0%; Mn: 0.2% to 2.0%; at least one element selected from the group consisting of Cr: 0.03% to 2.5%, Mo: 0.05% to 1.0%, Cu: 0.03% to 1.0%, Ni: 0.05% to 2.5%, Co: 0.01% to 1.0%, V: 0.01% to 0.5% and W: 0.005% to 1.0%, and satisfying a formula (2): C^(1/2)(1+0.7Si)(1+3Mn)(1+2.1Cr)(1+3.0 Mo)(1+0.4 Cu)(1+0.3Ni)(1+5.0V)(1+0.5W)>2.0   (2) wherein the balance is Fe and inevitable imurities.
 13. The method for making the component for machine structural use according to claim 10, wherein the steel material comprising, in terms of percent by mass, C: 0.3% to 1.5%; Si: 0.05% to 3.0%; Mn: 0.2% to 2.0%; at least one element selected from the group consisting of Cr; 0.03% to 2.5%, Mo: 0.05% to 1.0%, Cu: 0.03% to 1.0%, Ni: 0.05% to 2.5%, Co: 0.01% to 1.0%, V: 0.01% to 0.5% and W: 0.005% to 1.0%; at least one element selected from the group consisting of Ti: 0.005% to 0.1%, Nb: 0.005% to 0.1%, Zr: 0.005% to 0.1%, B: 0.0003% to 0.01%, Ta: 0.02% to 0.5%, Hf: 0.02% to 0.5% and Sb: 0.005% to 0.015%, and satisfying a formula (3): C^(1/2)(1+0.7Si)(1+3Mn)(1+2.1Cr)(1+3.0 Mo)(1+0.4 Cu)(1+0.3Ni)(1+5.0V)(1+1000B)(1+0.5W)>2.0   (3) wherein the balance is Fe and inevitable impurities.
 14. The method for making the component for machine structural use according to claim 10, wherein the steel material further comprising, in terms of percent by mass, at least one element selected from the groud consisting of S: 0.1% or less, Pb: 0.01% to 0.1%, Bi: 0.01% to 0.1%, Se: 0.003% to 0.1%, Te: 0.003% to 0.1%, Ca: 0.0001% to 0.01%, Mg: 0.0001% to 0.01% and REM: 0.0001% to 0.1%.
 15. (canceled)
 16. The component for machine structural use according to claim 4, the composition further containing, in terms of percent by mass, at least one element selected from among the group consisting of S: 0.1% or less Pb: 0.01% to 0.1% Bi: 0.01% to 0.1%, Se: 0.003% to 0.1%, Te: 0.003% to 0.1%, Ca: 0.0001% to 0.01%, Mg: 0.0001% to 0.01% and REM: 0.0001% to 0.1%.
 17. The component for machine structural use according to claim 5, the composition further containing, in terms of percent by mass, at least one element selected from the group consisting of S: 0.1% or less, Pb: 0.01% to 0.1%, Bi: 0.01% to 0.1%, Se: 0.003% to 0.1%, Te: 0.003% to 0.1%, Ca: 0.0001% to 0.01%, Mg: 0.0001% to 0.01% and REM: 0.0001% to 0.1%.
 18. (canceled)
 19. The component for machine structural use according to claim 4, wherein the composition further comprises 0.25% by mass or less of Al.
 20. The component for machine structural use according to claim 5, wherein the composition further comprises 0.25% by mass or less of Al.
 21. The method for making the component for machine structural use according to claim 12, wherein the steel further comprises 0.25% by amss or less of Al.
 22. The method for making the component for machine structural use according to claim 13, wherein the steel further comprises 0.25% by mass or less of Al. 